大型汽轮机的高中压缸采用双层结构，使汽缸内外很大的压力差由内缸和外缸分担，可以减薄内外缸法兰厚度，有利于汽轮机运行。（）
参考答案：对
— Could you help me decide which one to choose?
— Oh, _______ .[ ]
A. that's a good idea
B. maybe I can't do it
C. maybe it would be better to be up to you
D. go ahead, please.
This weekend we’re going to Beijing to attend a summer camp and we’ll visit the Bird Nest_______we get there.
A．when
B．if
C．once
D．until
