A circus act needs to select a juggler, a clown and an acrobat from a group of 10 performers. How many different choices of circus artists are possible if: (a) There are no restrictions. (b) Two of them (the 10 artists) will not perform together. (c) Two of them will perform together or not at all. (d) One of them must be in the act. (e) One of them can only perform as an acrobat.