1. In how many ways can 10 people sit in a row if
a. there are no restrictions
b. persons A and B must sit next to each other
c. there are 4 men and 6 women and the 4 men must sit next to each other
d. there are 5 sets of twins and each set of twins must sit together
e. there are 5 men and 5 women and no 2 men or 2 women can sit next to each other