离心泵轴封装置中依靠动、静环的贴合来密封的是（）。
A、填料密封
B、干气密封
C、浮环密封
D、机械密封
参考答案：D
________ people go abroad, they have to bring their passports with them.
A．Until
B．When
C．Because
D．Before
已知a，b∈R+，ab=9，则a+b的最小值是______．
