21. The most important thing about cotton in history is ________ part that it played in _______ Industrial Revolution.
A. /; / B. the; / C. the; the D. a; the
22. It hit me ______ my mother could be the proper person to persuade dad to change his mind.
A. which B. what C. that D. if
23. – I wonder how much you charge for your services.
-- The first two are free the third costs $30.
A. while B. until C. when D. before
