Billy Bob wants to gain some weight so that he can play football. Billy eats only milkshakes and spinach. Milkshakes cost him $1 each and spinach costs $2 per serving. A milkshake has 850 calories and a serving of spinach has 200 calories. Billy Bob never spends more than $20 a day on food and he always consumes at least 8000 calories per day. Which of the following is necessarily true?

(a) Billy Bob consumes at least 9 milkshakes a day.

(b) Billy Bob never consumes more than 6 servings of spinach a day .

(c) Billy Bob never consumes positive amounts of both goods.

(d) Billy Bob consumes only milkshakes.

(e) None of the above.