请大家帮我解答一下这道题，谢谢

usagmat

Each of the intergers from 0 to 9, inclusive, is written on a separate slip of blank paper and ten slips are dropped into a hat. If the slips are then drawn one at a time without replacement, how many must be drawn to ensure that the numbers on two of the slips drawn will have a sum of 10?

 

A.     3

B.     4

C.     5

D.     6

E.      7

谢谢。正确答案为什么是E。

tomrchu

pay attention to "ensure", so that means "must have sum of 10", so you have to draw: 0,1,2,3,4,5,6. since even when you reach 5, the biggest sum is still 9. so you have to draw anther one, which is "6"