The correct answer is C. since.
The word “since” is used to indicate a time period that begins at a specific point in the past and continues to the present. In this case, the speaker is saying that more than twenty years have passed since they had their first flight. This means that they had their first flight at some point in the past, and it has been more than twenty years since then.
The other options are not correct because they do not indicate a time period that begins at a specific point in the past and continues to the present. The word “after” is used to indicate a time period that begins after a specific point in the past. The word “while” is used to indicate a time period that overlaps with another time period. The word “when” is used to indicate a specific point in time.
Here are some examples of how the word “since” can be used in a sentence:
- I have been working here since 2010.
- I have not seen her since she moved away.
- I have been waiting for you since 10:00.