胎儿哪部分血管内血液的含氧量最低（）
A.静脉导管
B.脐动脉
C.下腔静脉
D.肺静脉
E.主动脉
参考答案：B
在公钥密码体制中，用于加密的密钥为______。
A．公钥
B．私钥
C．公钥与私钥
D．公钥或私钥
Your desk is crowded with too many unnecessary things，including used papers and books.You'd better ________ some of them.[ ]
A．remain
B．resist
C．remove
D．renew
