Question: 9 students, 5 men and 4 women, interview for 4 internships sponsored by a firm.
[A] In how many ways can the firm chose a set of four interns?
[B] In how many ways can the firm chose a set of four interns such that not everyone in a set is of the same sex?
[C] In how many ways can the firm chose a set of four interns if it must include two men and two women in each set?