矿用高强度圆环链分为（）三个等级。
A．A；
B．B；
C．C；
D．D；
E．E。
参考答案：B, C, D
The moon by human beings already. [ ]
A. was been visited
B. will be visited
C. had been visited
D. has been visited
恶性肿瘤通过血道转移最常受累的脏器是（）
A.脑、脊椎
B.脾、淋巴结
C.肺、骨
D.肝、脑
E.肝、肺
