21. You may take this book away _______you return it on time.
A. as soon as B. in case that C. so long as D. unless
22. —You ran into Mr. Li! How did that happen?
—Well, _______ of us was looking where we were going. We came around the corner at the
same time.
A. neither B. either C. both D. None
23. I supported him in time, otherwise he _______off the bike .
A．might fall B．would fall
C．would have fallen D．should have fallen
