[問題]考古Form23 Q25

[hibettychen]

Poor people, who laready puchase the cheapest fodd items - starches, frankfurters, beans, and the like - have no items that they can economize when there is a rise in the overall price of food.

(B) on which there can be economizing when there is a rise in the overall price of food
(C) for economizing if there will be an overall rise in the price of food
(D) on which to economize when overall prices of food rise
(E) that can be economical when overall prices of food rise

Ans: D

我想問答案(D) on which to economize when overall prices of food rise，紅色部分是如何變化而來的？為何關代後面可以接不定詞呢？
另外，請問(A)錯在哪裡？
謝謝解惑