Examine (a), (b), and (c) and select the correct answer.

(a) 50% of 20
(b) 30% of 90
(c) 25% of 80

a. 2(a) = (b)
b. (b) - (a) = (c)
c. (a) + (c) + (b)
d. 2(a) = (c)

(Explanation would be appreciated)