Four people-a, b, c, and d-are waiting to give blood. Of these four, a and b have type AB blood, whereas c and d do not. An emergency call has just come in for some type AB blood. If blood donations are taken one by one from the four people in random order and x is the number of donations needed to obtain an AB individual (so possible x values are 1, 2, and 3), what is the probability distribution of x?