[问题1] 什么是网络安全
参考答案：网络安全，就是用一组规则约束所有的网络活动，只有被允许的活动才能正常进行，所有不允许的活动都被禁止。
—I'm sorry. That wasn't of much help.
—Oh, ________. As a matter of fact, it was most helpful.[ ]
A. sure it was
B. it doesn't matter
C. of course not
D. thanks anyway
对CT空间分辨率的论述，正确的是（）
A.对一台机器而言，高于普通X线检查
B.对一台机器而言，有一定的极限
C.对一台机器而言，是一个定值
D.随着X线剂量增加而增加
E.与探测器大小无关
