Which of the following statements about the image formed by a concave lens is always correct?

The correct answer is B, Ovum. The female egg cell, or ovum, is the largest cell in the human body. It measures about 100 to 130 micrometres across, roughly the thickness of a hair, and is the only human cell that can just be seen without a microscope. It is this big because it carries the cytoplasm, food store and machinery an embryo needs in its first days, and a girl is born with her whole stock of these cells already formed. A is wrong because the sperm is the smallest human cell, a few micrometres in the head, built only to swim and carry a nucleus. C is wrong because a nerve cell is the longest cell, since a single axon can run about a metre from the spinal cord to the toe, but it is extremely thin and not the largest in volume. D is wrong because an osteoclast, the cell that dissolves bone, is large and has many nuclei yet stays smaller than the ovum. Exam tip: ovum is the largest cell, sperm the smallest and the nerve cell the longest.

The correct answer is B, dyne-sec/cm². The poise is the CGS unit of dynamic or absolute viscosity, named after the French physician Jean Poiseuille, and one poise equals one dyne-second per square centimetre, which is the same as one gram per centimetre per second. Dynamic viscosity measures the internal friction of a fluid, the shearing stress needed for a unit rate of shear, so its unit is a stress multiplied by time. In SI the unit is the pascal-second, and one poise equals 0.1 pascal-second. Water at room temperature has a viscosity of about one centipoise, while honey is thousands of times thicker. A is wrong because dyne/cm² is a unit of stress or pressure, with no time in it. C is wrong because gram is a unit of mass, and g-sec/cm² does not reduce to the dimensions of viscosity. D is wrong because gm-cm/sec² is the dyne itself, a unit of force. Exam tip: 1 poise = 1 dyne-s/cm² = 0.1 Pa-s; water is about 1 centipoise.

The correct answer is B, Geitonogamy. Geitonogamy is pollination between two different flowers borne on the same plant. It is an odd halfway case: because the pollen has to travel from one flower to another it needs an agent such as an insect or the wind, so in working it looks like cross-pollination, yet the pollen comes from the same plant, so genetically it is self-pollination and brings no new variation. A is wrong because autogamy is pollination within the same flower, where pollen falls on the stigma of that very flower, as in pea and wheat. C is wrong because xenogamy is the transfer of pollen to the stigma of a flower on a different plant, and it is the only one of the three that brings genetically different pollen and so gives rise to variation. D is wrong because option B already names the process correctly. Exam tip: autogamy – same flower; geitonogamy – same plant, another flower; xenogamy – another plant.

The correct answer is C, Carbohydrates. The buccal cavity is the mouth, and the saliva poured into it begins the digestion of starch, a carbohydrate. Three pairs of salivary glands supply saliva, which carries the enzyme salivary amylase, also called ptyalin. It works best in a slightly alkaline medium and breaks the long starch chains into the sugar maltose, which is why a piece of bread kept in the mouth slowly tastes sweet. The teeth and tongue help by grinding the food and rolling it into a bolus. A is wrong because protein digestion begins in the stomach, where pepsin works in the acid of hydrochloric acid. B is wrong because the digestion of fat takes place chiefly in the small intestine, where bile from the liver breaks fat into small droplets and pancreatic lipase acts on them. D is wrong because starch digestion clearly does begin in the mouth. Exam tip: mouth – starch by amylase; stomach – protein by pepsin; small intestine – fat by bile and lipase.

The correct answer is C, Yeast, amoeba, bacteria. All three are unicellular, which means a single cell carries out every job of life. In a unicellular organism that one cell manages nutrition, respiration, movement and reproduction, while a multicellular body shares the work among tissues and organs. Yeast is a single-celled fungus used in baking and brewing, amoeba is a single-celled protozoan that moves and feeds with pseudopodia, and bacteria are single-celled organisms whose cells have no true nucleus. A is wrong because hydra, though a simple animal, is multicellular, with a body wall and tentacles made of many cells. B is wrong for the same reason, as sea anemone is a multicellular animal of the phylum Cnidaria, even though paramecium and bacteria are single-celled. D is wrong because only paramecium in that group is unicellular; hydra and sea anemone are both multicellular. Exam tip: unicellular – bacteria, amoeba, paramecium, yeast, chlamydomonas; multicellular – hydra, sea anemone, spirogyra.