为了防止管线内水的倒流，在无人看护的地方，一般安装（）
A.闸阀
B.截止阀
C.单向阀
D.安全阀
参考答案：C
．We are going to ______ with some friends for a picnic . Would you like to join us?
A．get in
B．get over
C．get along
D．get together
为什么说民主化是最利于创造力发挥的首要条件？
