Treasury bills (T-bills) have short maturities and are sold at a discount, with the return being the difference between the purchase price and par value at maturity, without coupon interest. Marketable bonds, however, pay coupon interest. The feedback from the document states:

"Because T-bills have such short maturities, they do not pay any coupon interest; instead, they are sold to investors at a discount from par value. When the T-bill matures, you receive par value. The difference between the price paid and the par value represents your return."