A radio presenter has enough time at the end of their show to play five songs. She has 13 songs by four groups to choose from: five songs by The Anvils, four by The Braziers, three by The Chisels and one by The Dustbins. Find the number of ways she can choose five songs to play if she decides: 

a. That there should be no restrictions

b. To play all three songs by The Chisels

c. To play at least one song by each of the four groups.