Which would NOT be a good example of third-degree price discrimination?

a. Gas prices rising substantially before Fourth of July.
b. Attorneys and doctors having sliding scales for prices depending on client income.
c. Buying Tide laundry detergent at WalMart for a typical price, vs. buying Tide at Sam's Club (with its membership fee) at a lower price.
d. Different airfare and hotel rates for business vs. leisure travel.