Which of the following is the largest cell in the human body?

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.

The correct answer is A, Isotopes of Hydrogen. Hydrogen has three isotopes: protium with one proton and no neutron, deuterium with one proton and one neutron, and tritium with one proton and two neutrons. All three have atomic number 1, so they occupy the same place in the periodic table, but their mass numbers are 1, 2 and 3. Deuterium forms heavy water, used as a moderator in nuclear reactors, while tritium is radioactive and is used with deuterium in fusion research. Hydrogen is the only element whose isotopes are given separate names of their own. B is wrong because carbon's isotopes are written as carbon-12, carbon-13 and carbon-14. C is wrong because uranium's main isotopes are uranium-235 and uranium-238. D is wrong because titanium's isotopes carry no special names of this kind. Exam tip: protium, deuterium and tritium all have atomic number 1 and mass numbers 1, 2 and 3.

The correct answer is D, Whale. A whale is a mammal, not a fish, so it breathes with lungs and must come to the surface to take air through a blowhole on top of its head. It has no gills at any stage of its life. Whales are warm blooded, give birth to live young and feed them on milk, which is why they are grouped with dolphins and porpoises as marine mammals and not with fish; the blue whale is the largest animal known. A is wrong because an octopus draws water into its mantle cavity and passes it over gills. B is wrong because a squid, another cephalopod mollusc, also breathes through gills inside the mantle. C is wrong because a clown fish is a bony fish whose gills lie behind a bony flap called the operculum. Exam tip: whales, dolphins and porpoises are aquatic mammals — lungs and a blowhole, never gills.