眼表面的第一层保护层是（）
A．角膜上皮
B．结膜上皮
C．角膜和结膜上皮层
D．泪膜
E．眼睑
参考答案：D
The girl with two cats ______ in the yard when the earthquake happened. [ ]
A. was playing
B. is playing
C. are playing
D. were playing
某4B91型离心泵的流量为25升/秒，（介质为水，其比重为1kg/l）其扬程为91米，效率为68%，求轴功率？
