A student committee on academic integrity has 90 ways to select a president and vice president from a group of candidates. The same person cannot be both president and vice president. How many candidates are there?  Choose one.
A) 7  B) 8  C) 9  D) 10  E) 11

Rationale: "let the number of people to choose from = n no of people to be chosen = 2 the number of ways 2 people can be chosen from n people = nc 2 now , the two people selected can be arranged in 2 ! ways hence nc 2 * 2 ! = 90 which gives n = 10 answer : d"
Answer: D