Muscles move the limb away from the trunk, whereas muscles move the limb back toward the trunk.

The answers that fit the given blanks above are EXTENSOR and FLEXOR, respectively. Extensor muscles are those muscles that contract once extended and flexor muscles are those muscles that contract once flexed. When you extend, you move the limb away from the trunk; and when you flex, you are moving it toward the trunk.

The answer is a: the eggs have half as many chromosomes as the body cells