— ____ the Internet ____ in your school? 　

— Yes, but the computer in our office has often broken down. [ ]

A. Is, used　

B. Is, using　

C. Does, use

D. Has, used

门冬酰胺酶的抗癌作用机理是：

A．阻碍DNA合成

B．与DNA联结而破坏其结构

C．与DNA结合而阻碍RNA转录

D．阻碍纺锤丝形成

E．阻碍蛋白质合成