I believe the answer is C.
Being or amounting to a single unit or individual or entire thing, item, or object rather than two or more; a single
Most likely to entertain so C because it’s about memories
Answer:
How often do you review your lessons?
Explanation:
This is a question directed towards the other person. Therefore, "do you review" in the present tense is your best answer, as it is "up to the present point".
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