For question 21, the correct answer is C.  making learning experiences more engaging
For question 22, the correct answer is D. The word "where" is appropriate here as it indicates a location or environment where students feel comfortable
For question 23, the correct answer is C. The base form "evaluate" is needed here to match the imperative structure of the sentence
For question 24, the correct answer is A. "Although" introduces a contrast, which is appropriate for the context of discussing challenges and rewards in online teaching