A haulage contractor has two loads to deliver to separate customers. He has 4 trailers of which 2 are new. If the trailers are randomly chosen for any particular delivery:
(a) In how many ways can the trailers be allocated the deliveries?
(b) In how many ways can the trailers be allocated the deliveries such that exactly one new trailer is used?
(c) What is the probability that exactly one new trailer is used?