如果2x3ym与8xn+6y2是同类项，则nm=（）．

Don't spoil the child. Can't you make your little son _______ himself?

A．behave

B．believe

C．perform

D．conduct

﹣1⁴﹣（2﹣3）²×（﹣2）³

由0，1，2，…，9这十个数字组成的、无重复数字的四位数中，个位数字与百位数字之差的绝对值等于8的个数为（    ）个。

— Please _____ us in the discussion.

— I'd like to, but I'm _____ busy at the moment. [ ]

A. attend; too

B. join, far too

C. take part in; much too

D. join; too much

以下关于储金球的说法哪项是错误的（）

A.可抵偿合金凝固缩短

B.可避免铸件缩孔

C.直径略年夜于铸件蜡型最厚处

D.位置距蜡型1mm

E.应在铸圈的温度中心区