采煤机开启时应先将（）转起来，再给牵引速度。
A.滚筒
B.刀齿
C.销轨轮
参考答案：A
已知菱形的两条对角线的长分别是8和6，则该菱形的周长是______．
I_______ my homework in my study the whole morning . I went nowhere.
A．do
B．did
C．was doing
D．would do
