胎具组装法是利用单个独立的组装夹具进行装配的方法。( )
参考答案：错
管输天然气中的机械杂质对压缩机和燃气发动机影响很大。
— __ has your friend been to the Great Wall?
—Twice.[ ]
A. How often
B. What time
C. How long
D. How many times
