In producing a sweater, a man who shears sheep pays a farmer 4 for a sheep's wool. The shearing shop sells the wool to a knitting mill for 7. The knitting mill makes it into a fine fabric and sells it to a sweater-making firm for 13. The sweater-making firm sells the sweater to a clothing store for 20, and the clothing store sells the sweater, gift wrapped, for 50. What is the contribution to GDP of the production and sale of the sweater?

a. 94

b. 4

c. 44

d. 50