A teacher of 6 students takes 2 of his students at a time to a zoo as often as he can, without taking the same pair of children together more than once. How many times does the teacher go to the zoo?  Choose one.
A) 12	B) 51	C) 13	D) 15	E) 17
Two students can be selected from 6 in 6C2 =15 ways.   Therefore, the teacher goes to the zoo 15 times.   Ans: 15
<result>D</result>
A teacher of 6 students takes 2 of his students at a time to a zoo as often as he can, without taking the same pair of children together more than once. How many times does the teacher go to the zoo?