Bob views apples and oranges as perfect substitutes in his consumption, and MRS = 1 for all combinations of the two goods in his indifference map. Suppose the price of apples is $2 per pound, the price of oranges is $3 per pound, and Bob's budget is $30 per week. What is Bob's utility maximizing choice between these two goods?
选项：

A:4 pounds of apples and 6 pounds of oranges
B:5 pounds of apples and 5 pounds of oranges
C:10 pounds of oranges and no apples
D:15 pounds of apples and no oranges
E:none of the above