Which of the following is a carnivorous plant?

The correct answer is C, Germanium. Germanium is not used to build light emitting diodes because it is an indirect band gap semiconductor, so an electron falling back across the gap gives up most of its energy as heat in the crystal rather than as light. Its band gap is also very small, about 0.66 electron volt, which is far too little to produce visible light. Light emitting diodes are therefore made from direct band gap compounds, whose gap can be tuned to the colour wanted. A is wrong because gallium phosphide is used for green and yellow diodes. B is wrong because indium gallium nitride gives the blue and white diodes that made modern lighting possible and won the Nobel Prize in Physics in 2014. D is wrong because gallium arsenide is the standard material for red and infrared diodes, including the ones inside television remote controls. Exam tip: light emitting diodes need direct band gap materials such as gallium arsenide, gallium phosphide and indium gallium nitride; silicon and germanium are indirect and do not emit light.

The correct answer is C, At room temperature, surface tension of water is less than ethyl alcohol. This is the statement that is not correct, because water has a much higher surface tension than ethyl alcohol. At about twenty degrees Celsius water measures close to seventy two dynes per centimetre while ethyl alcohol is only about twenty two, since the hydrogen bonds between water molecules pull far more strongly than the weaker forces inside alcohol. That is also why alcohol spreads out thinly on a surface while water gathers into beads. A is a correct statement, because surface tension is expressed in dynes per centimetre in the older unit system and in newtons per metre in the SI system. B is correct, because the inward pull on the surface gives a small free drop the shape with the least area for its volume, which is a sphere. D is correct, because surface tension arises from the cohesive forces among the molecules of the liquid itself. Exam tip: water is about 72 dynes per centimetre and alcohol about 22; surface tension falls as temperature rises.

The correct answer is B, Villi. Villi are the small finger-like folds on the inner wall of the small intestine, so they belong to the digestive system. Their work is to enlarge the surface through which digested food is absorbed; each villus does hold a capillary and a lymph vessel inside it, but the villus itself is not a part of the circulatory system. The human circulatory system is made of the heart, the blood and the blood vessels. A is wrong because capillaries are the thinnest vessels, where gases and nutrients pass between blood and the tissues. C is wrong because veins carry blood back to the heart and have valves to stop it flowing backwards. D is wrong because arteries carry blood away from the heart and have thick elastic walls to take the pressure. Exam tip: villi absorb digested food in the small intestine; the heart, blood and blood vessels make up the circulatory system.

The correct answer is A, To protect the lining of the stomach. The gastric glands in the stomach wall release three things: hydrochloric acid, the enzyme pepsin and mucus. The acid makes the contents strongly acidic so that pepsin can act, and the mucus coats the inner wall as a thick slippery layer so that this acid and the enzyme do not digest the stomach's own tissue. When the mucus layer breaks down the acid attacks the wall and an ulcer forms; the bacterium Helicobacter pylori is a common cause. B is wrong because fat is digested by lipase, helped by bile from the liver, mainly in the small intestine. C is wrong because germs in food are killed by the hydrochloric acid and not by mucus. D is wrong because protein is broken down by pepsin in the stomach and by trypsin further along. Exam tip: gastric juice — HCl for acid, pepsin for protein, mucus for protection of the lining.

The correct answer is D, It has no moving parts. A solar cell turns sunlight straight into electricity by the photovoltaic effect at a semiconductor junction, usually of silicon, so nothing inside it turns or slides. Having no moving parts means it needs almost no maintenance, makes no noise and lasts for decades once installed, which is why panels are used on satellites, in lighthouses and in village homes far from any grid. A single silicon cell gives about 0.5 to 0.6 volt, so cells are joined into modules and modules into panels for useful power. A is wrong because a solar cell shows no oscillatory motion; it is a static device. B is wrong because metal on the outer area describes the contacts and is not an advantage. C is wrong because having moving parts is the opposite of the cell's real strength. Exam tip: solar cell — photovoltaic effect in silicon, no moving parts, about 0.5 V per cell.