A welder orders argon gas and pure oxygen. The distributor requires a minimum order of $250. A cubic foot of argon gas costs $0.75 and a cubic foot of pure oxygen costs $0.50. To summarize the situation, the welder writes the inequality:

0.75a + 0.5p ≥ 250, where a is the number of cubic feet of argon gas and p is the number of cubic feet of pure oxygen.

Which graph's shaded region shows the possible combinations of argon gas and pure oxygen the welder can buy?