He took()my job when I was on holiday.
A.on
B.over
C.off
D.after
参考答案:B
写出方程x1+x2+…+x1998=x1x2…x1998的一组正整数解______.
阀门阀芯材料应比阀座材料(),这样才能避免阀座密封面在研磨过程中产生()或()。