21. Mr. Morgan gradually ?____? a knowledge of the subject.?
A. attainedB. requiredC. acquiredD. enquired?
22. Though steam engines have now passed the ?____? of their usefulness, their heritage remains.?
A. primeB. eraC. epochD. date?
23. What he said just now had little to do with the question ?____?discussion.?
A. withB. underC. byD. over?
24. Molten iron is poured into the mixer in the same way ?____?tea is poured into a cup from a teapot.?
A. asB.thatC. sinceD. like?
25. The Russians also burned their farms and ?____? their dams and power plants.?
A. blew upB. blew offC. blew outD. blew over