The events A and B are independent. The probability that event A occurs is p and the probability that event B occurs is q. Which of the following is equal to

**the probability that exactly one of the events A and B occurs**?

A. p − 2pq

B. q − pq

C. p + q − 2pq

D. p + q − pq

E. p + q

The answer is C. But, the part that I am having trouble understanding is the bold statement above: the probability that exactly one of the events A and B occurs...isn't that equivalent to "the probability that either A or B occurs? In other words, P(A or B), which would make choice D a correct answer?