题目

______ it ______ for your help, I couldn’t have made any progress. A. Had; not been                  B. Should; not been C. Did; not been                   D. Not; been 答案：A讨论下述函数的奇偶性：（1）f（x）=，（2）f（x）=，（3）f（x）=，（4）f（x）=（常数a≠0）．