ＰＳ1問教えてください

下記の問題を教えてください。

A magazine distributor finds that for every dollar she reduces the subscription rate of a certain magazine, she increases her weekly sales by one hundred subscriptions. If she currently sells 500 subscriptions per week at a $10 subscription rate, at which of the following subsccription rates would she maximize her earnings?

(A) $10
(B) $9
(C) $7　　→正解
(D) $5
(E) $2

意味がよくとれません。
特に、”for every dollar she reduces the subscription rate of a certain magazine, she increases her weekly sales by one hundred subscriptions”です。