Two fair coins are being used in a probability experiment. During each trial of the experiment, the
 coins are flipped and the outcomes are recorded with H representing heads and T representing tails.
 There are four possible outcomes: HH. HT. TH, and TT. What is the probability, on each trial of the
 experiment, that the outcome is one head and one tail, in any order?
 25%
 33%
 50%
 75%