Read the text and match the headings (A-G) to the paragraphs (1-5). There are two ex headings you do not need to use.
1)
Biological clock is a popular name given to a mysterious timing system that operates in plants and animals. It keeps accurate time of days, weeks, months, and even years. It also times the activities of living things to keep them in harmony with the regular changes of their surroundings. Birds migrate, fish spawn, and flowers blossom on schedules set by their built-in clocks. The biological clock in human beings times our periods of sleep and wakefulness and many body activities.
2)
No one knows where this biological clock is located, or how it works. Some scientists believe that every living thing has its own built-in timing system that operates independently. Others believe that the clocks ae set by the natural rhythms of the earth's electromagnetic or gravitational forces. Still others suggest that eep the clocks accurate. Scientists hope that experiments with plants and animals in outer space, far from the earth's natural rhythms, may provide more information about how biological clock works.
3)
Biological clocks keep track of rhythmic changes in the environment, including day and night, movements ol the ocean tides. phases of the moon, and seasons of the year. Many, and possibly all, living things seem to have internal cycles - called biorhythms- that respond to these external rhythmic changes. The biorhythms of each particular species seem to be timed to enable the organism to take advantage of the changes in its environment.
4)
Biorhythms continue on schedule even in laboratories where the plant or animal is shielded irom all evidence of passing time and ol outside change. But the biorhythms can be shifted- and the biological clock reset by changing the time at which the plant or animal gets light.
5)
Biological clocks in people work on schedules essential to life and health. Many vital body processes ire programimed for 24-hour biorhythms. The lunctions of cells, glands, kidneys, liver, and nervous system are al coordinated with each other and with the day-night rhythm of the environment. Each is timed to operate when t can do the greatest good for the entire body.

A. Location of biological clock B Internal.cycles and rhythmic changes
C Bological clock and changing
D. The clock of the body
E. Coordinated work of the body F. The internal clock's research
G. Body harmony and environment
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