While burning hydrocarbon fuels, if we see a yellow flame with lots of black smoke, it means that the fuel is:

The correct answer is A, parallel to each other. A simple periscope has two plane mirrors facing each other in parallel, each tilted at 45° to the tube.

Light from the object enters the top opening and strikes the upper mirror, which is set at 45° and turns the light through 90° down the tube. The lower mirror, parallel to the first and also at 45°, turns the light through another 90° so that it travels horizontally into the observer's eye. The image is upright and the same way round as the object, because the two reflections cancel each other's lateral inversion. Periscopes let soldiers in trenches and crews of submarines see over obstacles without exposing themselves.

B is wrong: perpendicular mirrors would send the light back the way it came instead of down and out of the tube.

C is wrong: mirrors at 60° to each other form multiple images, as in a kaleidoscope, and are not used in a periscope.

D is wrong: 45° is the angle each mirror makes with the tube, not the angle between the two mirrors.

Exam tip: periscope = two parallel plane mirrors at 45° to the tube; kaleidoscope = mirrors at 60°; number of images between two mirrors = 360/θ − 1.

The correct answer is B, All variations in a species have equal chances of survival. This is false, because variations differ in how well they suit the environment, and that difference is the basis of natural selection.

Variation means differences among individuals of a species. In asexual reproduction offspring are near copies of a single parent, so variation is minimal and arises only from small errors in copying DNA. Sexual reproduction mixes genes from two parents and creates much more variation. Changes in the genetic make-up, through mutation and recombination, are the source of heritable variation. The environment then favours some variants over others: Darwin's example of beetles shows that a colour which hides them from crows survives and spreads, while others are eaten. Unequal survival is therefore the essence of evolution.

A is wrong: the statement is true, since asexual offspring are nearly identical.

C is wrong: the statement is true; genetic change is the source of variation.

D is wrong: the statement is true; environmental factors select the variants that survive.

Exam tip: variation + natural selection = evolution; asexual reproduction gives the least variation, sexual reproduction the most.

The correct answer is B, Mild base. Milk of magnesia is a suspension of magnesium hydroxide, a mild base that neutralises excess stomach acid without harming the body.

The stomach secretes hydrochloric acid to digest food. When too much is produced, it causes acidity, indigestion and a burning feeling. An antacid such as milk of magnesia, Mg(OH)2, reacts with the extra acid to form magnesium chloride and water, a neutralisation reaction that brings relief. A strong base like sodium hydroxide would burn the tissues of the mouth and stomach, so only a weak, sparingly soluble base is safe to swallow. Milk of magnesia is also used as a mild laxative.

A is wrong: a strong base would damage the digestive tract.

C is wrong: an acid would add to the problem, which is already too much acid.

D is wrong: a mild acid also cannot neutralise acidity; it would only make it worse.

Exam tip: antacids are mild bases such as magnesium hydroxide and sodium bicarbonate; stomach acid is HCl with a pH of about 1.5 to 3.5.

The correct answer is A, 3.00. Energy equals power multiplied by time: 0.1 kW × 30 hours = 3 kWh, which is 3 units.

The bulb's power is 100 W, which is 0.1 kW. It runs 10 hours a day for 3 days, a total of 30 hours. Electrical energy consumed = power × time = 0.1 kW × 30 h = 3 kWh. One kilowatt-hour, the commercial unit of electrical energy on electricity bills, is the energy used by a 1,000 W appliance running for one hour. In SI units, 1 kWh equals 3.6 × 10^6 joules, so this bulb uses 1.08 × 10^7 joules over the three days.

B is wrong: 1.08 comes from the figure in joules (1.08 × 10^7 J), not from kWh.

C is wrong: 2.16 does not follow from 100 W, 10 hours and 3 days.

D is wrong: 0.33 would be the result of dividing instead of multiplying.

Exam tip: units (kWh) = watts × hours ÷ 1000; 1 kWh = 3.6 × 10^6 J; 1 horsepower ≈ 746 W.

The correct answer is C, proteins. Chromosomes are made of DNA wound around proteins, mainly histones; together this DNA-protein material is called chromatin.

In eukaryotic cells, a very long DNA molecule wraps around groups of eight histone proteins to form beads called nucleosomes. These coil and fold further, so that about two metres of DNA fits inside a nucleus only a few micrometres wide. During cell division the chromatin condenses into the compact, visible chromosomes. Each chromosome carries many genes. Human body cells have 46 chromosomes in 23 pairs, of which one pair, XX in females and XY in males, decides sex.

A is wrong: lipids form cell membranes, not chromosomes.

B is wrong: vitamins are micronutrients that help the body's chemical reactions and are not structural parts of chromosomes.

D is wrong: carbohydrates are energy sources and cell-wall materials; the sugar deoxyribose is part of DNA itself, but chromosomes are described as DNA plus protein.

Exam tip: chromatin = DNA + histone proteins; nucleosome = DNA wrapped on eight histones; humans have 23 pairs of chromosomes.