The answer is C.) Sac. More formally known as the amniotic sac. It's also sometimes called the bag of waters.
Reflections and absorption by the atmosphere prevent some radiation from reaching Earth's surface.
Pituitary gland.
When you feel stress, the pituitary gland, at the base of the brain, increases its production of the hormone ACTH. This hormone tells the adrenal glands, found at the top of your kidneys, to increase their production of hormones. These stress hormones help you to concentrate, speed your reaction time, and boost your strength. Your hypothalamus also helps your body respond to stress.
A tropism.
A plant movement to a light stimulus is a phototropism.
A plant movement to water stimuli is a hydrotropism.
etc. etc. :)