题目

If he had started at nine o’clock, he _____ have been in London by eleven o’clock. A. must                     B. may                      C. ought to          D. has to 答案：C4．下列式子中的y是x的函数吗？为什么？并求出取值范围．（1）y=3x-5；（2）y=$\frac{x-2}{x-1}$；（3）y=$\sqrt{x-1}$．