_____ in this university that only those who have passed CET4 can graduate.                    [ ]

A. What is required

B. What requires

C. It requires

D. It is required

引起着色性干皮病的原因是（）。

A.DNA上TT二聚体的切除修复系统有缺陷

B.细胞膜通透性缺陷引起迅速失水

C.紫外线照射诱导了有毒性的前病毒

D.温度敏感性转移酶失活

E.维生素B₁缺乏