21. _______, let me introduce my friends to you.
A. In all B. After all C. Above all D. First of all
22. I’ll never forget the days_______ I spent on the farm.
A. when B. that C. in which D. in that
23. His mother told him not to stay up, ______ would be bad for his health.
A. which B. that C. what D. it
