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Fill in each gap with one of the following words. (There is an extra word.) Then check your answers with the help of the key below.


who / why / until / from / is / to / may / with / that / as / and / of / are / most / at /




Every human body ages over time. The human life span varies __________ (1) person to person. On average, a newborn today can expect to live to the age __________ (2) about 75. Scientists believe that the probable maximum human life span __________ (3) about 150 years; the record of the oldest person to date is Shigechiyon Izumi (1871 - 1986) of Japan, __________ (4) lived to be 120 years and 237 days.

There are a variety of theories as to __________ (5) all living things grow old and die. The free-radical theory states __________ (6) free radicals, certain chemicals produced as a by-product of biological activity, __________ (7) particularly harmful to healthy cells. As a person ages, free radicals gradually destroy cells __________ (8) they can no longer function properly, causing the entire body (especially whole organ systems such __________ (9) the kidneys or heart) to break down __________ (10) die. The programmed senescence theory suggests that the rate __________ (11) which we age is predetermined, and that our genetic makeup controls the aging and death of the cells. After enough of the cells die, the organs cease __________ (12) function and death occurs.

Many researchers also believe that the average life spans of __________ (13) species are predetermined. If this is true, it may be that a species' life span encourages the survival of that species. For example, humans __________ (14) live only a certain length of time so that the young can introduce new combinations of traits, thereby strengthening the human gene pool.


Go down to find the key to this exercise.









Key to the exercise:


1: from

2: of

3: is

4: who

5: why

6: that

7: are

8: until

9: as

10: and

11: at

12: to

13: most

14: may