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...

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?