多项式ab﹣2a﹣3的系数和常数项分别是[ ]
A．ab，﹣2a，﹣3
B．0，﹣2，﹣3
C．1，﹣2，﹣3
D．1，2，3
答案：C
The two little girls _____ good friends but for the attack of some outside force. [ ]
A. could have been
B. had been
C. were
D. must be
操作式条件反射
