请教nn一道语法题...谢谢:)

Since chromosome damage may be caused by viral infections , medical x-ray, and exposure to sunlight , it is important that the chromosome of a population to be tested for chemically induced damage be compared with those of a control population.

A to be tested for chemically induced damage be compared with

B  being tested for damage induced chemically are compared with

C  being tested for chemically induced damage should be compared to

D  being tested for chemically induced damage are to be compared to

E  that is to be tested for chemically induced damage are to be comparable  with

  能解释下为什么是A吗?

这里应该是虚拟.

记得tonyadidas说过it is important that 是可以有虚拟的用法出现的,在OG上有体现