21.This is a simple idea, but___________ which is hard to put ________ into practice.
A .one; x B. the one; it C. that; it D. one ; it
22. The project along with the experiment we had once devoted so much time _________ perfectly.
A .to work out B. to working out C. to worked out D. has worked out
23. Most US citizens retire around the age of 62, __________ is the national minimum retirement age.
A . when B which C. it D. that
24. I forget where I read the article, or I ________ it to you now.
A . will show B. would show C. am going to show D. am showing
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