Idiomatic and Phrasal Usage with Preposition

Directions: This question presents a sentence, all or part of which is underlined. Beneath the sentence you will find five ways of phrasing the underlined part. The first of these repeats the original; the other four are different. If you think the original is best, choose the first answer; otherwise choose one of the others.

She found the subject difficult at first, but now she is really getting off her stride.