单项选择题

The safety of mobile phones (U. S. cell-phones) seems to face new questions after British researchers found that microwave emissions can affect the health of earthworms in unexplained ways.
The larvae of tiny soil worms called nematodes grew faster and became more (1) after they were (2) for a long time to weak microwave radiation, of the (3) strength and frequency as that (4) by mobile phones, they found.
Why this occurred is (5) , and there is no evidence that human health is also affected. But, they warn, the finding challenges the standards which (6) whether a mobile is safe or not.
These standards are based on whether the radiation is strong enough to cause a batch of (7) to warm up. Heat causes the chemical bonds in cells to (8) , a disruption that can be a potential (9) for cancer.
The research, led by David de Pomerai at Nottingham University, central England, said larvae exposed to the microwaves (10) between eight and eleven percent faster than (11) which had not been exposed.
In addition, the (12) of worms that (13) into egg-bearing adults was 28～40 percent higher.
By comparison, worms that had been exposed to (14) heat suffered a 10-percent reduction in their growth rate, and (15) none developed into fertile adults.
The experiment is important because it suggests that microwaves might trigger biological effects. (16) warming any tissues.
The scientists do not (17) out the possibility that radiation could cause hot-.spots to occur somewhere in the worm that then affected cell (18) .
The worry, though, is that the 19 basis for determining mobile safety is whether microwaves cause heating in bulk tissue rather than just at (20) in it, they note.

20（）

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