1. Since chromosome damage may be caused by viral infections, medical x-rays, and exposure to sunlight, it is important that the chromosomes 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（A）
(E) that is to be tested for chemically induced damage are to be comparable with
以前好象看懂过，现在怎么又不明白了，为什么要用虚拟语气？ | 
