Q31: The numbers x and y are three-digit positive integers, and x + y is a four-digit integer. The tens digit of x equals 7 and the tens digit of y equals 5. If x < y, which of the following must be true?

I. The units digit of x + y is greater than the units digit of either x or y.

II. The tens digit of x + y equals 2.

III. The hundreds digit of y is at least 5.

A. II only

B. III only

C. I and II

D. I and III

E. II and III

答案是B,我怎么觉得是E啊!条件II 错了吗?还是我理解有问题?请指教

条件二错。原因是：The tens digit of x equals 7 and the tens digit of y equals 5.

但是你忽略了x和y的个位相加可以>10，也就是The tens digit of x + y equals 2.或者3