Suppose that a coin is tossed 4 times. Each time the result H for heads or T for tails is recorded. An outcome of HHTT means that heads were obtained on the first two tosses and tails on the second two. Assume that heads and tails are equally likely. a) How many distinct outcomes are possible? b) What is the probability that exactly two heads occur? c) What is the probability that exactly one head occurs?