Here are the corrections for the sentences:

21. **hardly** should be corrected to **hard**.
Correct sentence: She cried very hard when she heard the news of the accident.

22. **when** should be corrected to **since**.
Correct sentence: It has been a long time since I last wrote to you.

23. **is smelling** should be corrected to **smells**.
Correct sentence: The food that my mother is cooking in the kitchen smells delicious.

So the corrections are:
- 21: B
- 22: C
- 23: C