Biological Clock
When the sun rises on a warm and sunny day, you can see some flowers open up. When they close late in the afternoon, other flowers open.
Biological rhythms（生物节律）, like the opening and closing of flowers, happen all over nature. But not all of them are daily rhythms. Some, like the beating of our heart, happen every second.______（26）
Scientists use the term biological clock to describe the timing that controls biological rhythms.______（27）In some animals it is probably controlled by the brain. In plants and other living things that have no brain, it must be something else.
The migration of animals happens when a signal is sent out. For example, when days become shorter, birds leave the northern parts of the world and fly south where it is warm.______（28）
Humans also have biological clocks that control their daily rhythms. When people travel by plane from one continent to another, they often cross many time zones. Their internal clocks don’t seem to work correctly. We call this jet lag（时差）.______（29）People who work night shifts also have problems with their biological clocks. They may not be as active as people who work during the daytime.
Biological clocks control many rhythms of life.______（30）Doctors are looking for new ways to make travelling more comfortable and medical treatment more effective.
（28）_____
A、But where can you find it?
B、This makes them feel tired.
C、In the summer they fly back to have babies.
D、We are learning more and more about them.
E、Biological clocks can be adjusted.
F、Other rhythms are based on months, seasons or years.
【正确答案】：C
【题目解析】：空格前的一句话意思为“例如，当白天时长变短时，候鸟们就离开北方地区，飞往温暖的南方地区。”这里讲的是候鸟如何迁徙，那么选项C（在夏天它们又飞回来繁殖后代）与前一句话的意思衔接最为紧密。故选C。
