______known to us ,_______ increasing number of students are buried in their studies.[ ]

A.What is;a

B.It is;the

C.As it is;the

D.As is;an

低水平放射性测量时为把简化优质因子提高100倍，本底计数率应降到多少（设其他参数不变）（）。

A.1／10

B.1／30

C.1／50

D.1／100

E.1／200