Determine whether each of the following pairs of events is mutually exclusive.
a. Five coins are tossed: “one head is observed,” “at least one head is observed.”
b. A salesperson calls on a client and makes a sale: “the sale exceeds $100,” “the sale exceeds $1000.”
c. One student is selected at random from a student body: the person selected is “male,” the person selected is “older than 21 years of age.”
d. Two dice are rolled: the total showing is “less than 7,” the total showing is “more than 9.”