Momodou has been ill … three weeks now.

Analyze the question: The sentence is "Momodou has been ill ...... three weeks now." It uses the present perfect continuous/perfect tense ("has been"), and the blank is followed by a length of time ("three weeks").

Explain the concept: With the present perfect tense, we use two key prepositions of time:

• since is used with a specific starting point in time (e.g., since Monday, since 2010, since last week).
• for is used with a duration or period of time (e.g., for three weeks, for two years, for a long time).

Show the solution:

1. "Three weeks" is a period of time (a duration), not a starting point.
2. So we need for, which pairs with durations.
3. Checking the other options: • since: would need a starting point, like "since three weeks ago" (so "since three weeks" is wrong). • through: means from beginning to end of something (e.g., "through the night"), and does not fit here. • before: shows an earlier time and does not fit the present perfect tense with a duration.

Correct sentence: Momodou has been ill for three weeks now.

Answer: The correct option is B. for.

Tip: Remember: "Since" = Start point, "For" = Frame of time (duration). If you can answer "how long?" with a number of days, weeks, or years, use for. If you can name a date or event, use since.