statistics homework

A class has 20 students. The classroom consists of 20 desks, with 4 desks in each of 5 different rows. Amy, Bob, Chloe and David are all friends, and would like to sit in the same row. Answer each of the following questions and justify your reasoning. 

a. how many possible seating arrangements are there (assuming any student can sit where they want to)

b. how many possible seating arrangements are there such that amy, bob, chloe and david are in the same row?

c. What is the probability that if the students were assigned a random seating arrangement, that amy, bob, chloe and david end up in the same row as one another?