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.
（29）_____
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】：B
【题目解析】：空格前的三句话意思为“当人们乘飞机跨越大洲进行旅行时，往往会穿过很多时区。他们自身的时钟似乎无法正常工作。我们称之为时差。”选项B（这让他们感觉到疲惫）是对时差对人的影响的陈述，衔接了上下文。故选B。
