Each of the integers 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
the answer is e. mine is 8,not 7. | 
