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.
II only
III only
I and II
I and III
II and III
X是a7b,y是c5d,假设个位数进一,x+y的十位数是3,进1.
a+c大于等于9,因为x十位数大于y的十位数,所以百位必定小,c至少为5
tenth of (x+y)>=2.