Many students asked the teacher _____ the time of the test.
[ ]
A. for
B. to
C. about
答案:C
AD为△ABC的高,AB=AC,△ABC的周长为20cm,△ACD的周长为14cm,则AD=( )
紧定螺钉和()均可起到周向定位和固定作用。
A.键
B.花键
C.轴端挡圈
D.销