阅读下列短文， 从给下面每小题做给的A、B、C、D四个选项中， 选出最佳选项
Why are the leaves on trees different colors? What makes them change color? Leaves use sunlight, water, and air to make food for the trees. This way of making food is called photosynthesis. Photosynthesis is important to animals as well as plants. Some animals ea t plants. Other animals eat these animals.
Summer is playtime. For the leaves of many trees, however, it's work, work, work! The Sun is shining. The leaves are making food for the trees. The trees keep more than enough food deep inside. Leaves are green in summer because they are making food. Leaves also have orange, yellow, or brown colors inside them. During summer, those colors are covered by the green color.
Many places get colder in fall with shorter days and longer nights. Trees get less sunlight, so photosynthesis slows down. Over time, leaves stop making the green chemical. The hidden orange, yellow, and brown colors grow when that happens.
Once it gets cold enough and ice appears at night, the trees drop their leaves. They do this to keep themselves safe. If the leaves turn to ice while still on the trees, the trees can be hurt. Trees that drop their leaves are bare (光秃秃) in winter. They use the food they save in summer to get through the cold days and nights.
Spring comes before long. Ice and snow disappear and the rains come. The Sun shines, and tiny leaves grow from branches. Once again, the leaves use sunlight, water, and air to make food. The cycle(循环) begins again.
