The OA on old questions can be tough because the GMAT changes over time. OAs on questions from test prep companies can be even less reliable because we are mimicking the GMAT as best we can, but we don't always get it right.
Let me say this, there is NO WAY the GMAT would have C be the correct answer 'today'. Exchange 'with' may be the technically correct idiom, but exchange 'for' is acceptable and there is virtually no meaning difference between the two forms, so that issue isn't one that the GMAT will be using for eliminations. The issues of redundancy and tense shift are solid issues that the GMAT would use to invalidate answer choice C.
For the most part, you can rely on the explanations from test prep companies. In this case, however, the OA does not represent how the GMAT will be testing you today.
KW