离心泵汽蚀破坏主要发生在（）。
A．叶片进口背面靠前盖板处
B．叶片进口正面靠前盖板处
C．A或B
D．叶轮外缘叶片及盖板、蜗壳或导轮处
参考答案：D
— How wonderfully you are playing the piano! do you practice it a week?
— Twice. Practice makes perfect.
A. How often B. How many times C. How soon
循环经济
