这5道英文选择题到底选哪个?

这5道英文选择题到底选哪个?
16. Luxun and his works ___ we are familiar with are considered great.
A. whoseB. whoC. whichD. that
17. ___ it will do us harm remains to be seen.
A. What B. Whatever C. Whether D. That
18. I don’t know if he ________, but if he ________ I will let you know.
A. comes, comesB. will come, will comeC. comes, will come D. will come, comes
19. You can contact us by telephone or e-mail, _________ you prefer.
A. whenever B. however C. whichever D. whatever
20. This is a very useful book. I’ll buy it, _________.
A. how much may it cost B. no matter how it may cost
C. however much it may costD. how may it cost