You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.

[EX Q]: Question: Which of the following is NOT a factor of 10! ?

 Option A: 1440

 Option B: 625

 Option C: 160

 Option D: 80

 Option E: 50
[EX A]: Option B

[EX Q]: Question: A boy and a girl appear for an interview against 2 vacant posts in an office. The probability of the boy's selection is 1/5 and that of the girl's selection is 1/3. What is the probability that only 1 of them is selected?

 Option A: 1/3

 Option B: 2/5

 Option C: 3/5

 Option D: 4/7

 Option E: 5/9
[EX A]: Option B

[EX Q]: Question: If x is a positive integer, which of the following must be odd?

 Option A: x+1

 Option B: x^2+x

 Option C: x^2+x+1

 Option D: x^2−1

 Option E: 3x^2−3
[EX A]:
Option C