In context, which of the following revisions must be made to sentence 1 (reproduced below)?
Taking great notes is one of the best ways to ensure a high grade on a test or exam.
(A) Add “Usually” to the beginning of the sentence.
(B) Add “definitely” after the word “is.”
(C) Change “to ensure” to “of ensuring.”
(D) Change “grade” to “mark.”
(E) Add “normally” to the end of the sentence
The correct answer is (C) but "to ensure" and "of ensuring" both sound right to me. Can someone please explain why the revision was implemented.