在管线与铁路或道路交叉时,其交角不宜小于( )。
A.90°
B.75°
C.45°
D.30°
参考答案:D
The impact of Einstein’s breakthrough in the field of photo electric effect ____ the understanding of the universe was simply tremendous.
A.in
B.on
C.with
D.about
功率是60kW的汽车以36km/h的速度匀速行驶90s,它共做了______J的功.两台机器的功率之比Pl:P2=2:1,做功时间之比t1:t2=1:3,则这两台机器做功之比W1:W2=______.