Biological ClockWhen the sun rises on a warm and sunny day, you

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.

（30）_____


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】：D
【题目解析】：空格前的一句话意思为“生物钟控制着多种生活节律”，后一句话意思为“医生们正在寻找新的方法来使旅行变得更加舒适，医疗变得更为有效。”这说明我们正在逐步了解生物节律对我们的影响，并开始研究如何减小它对我们生活的影响。选项D（我们对这些生物节律了解得越来越多）最为合适。故选D。