下列选项不属于截止阀特点的是（）
A.流体阻力大
B.介质流动方向受限制
C.流体阻力小
D.启闭力矩大
参考答案：C
Before her first Harry Potter came out in 1997，J.K.Rowling ________ English for a living in Portugal.
A．is teaching
B．had taught
C．has taught
D．would teach
（3分）下表列出了几种键的键能:
。
