12. Not until the activists arrived at the site _____ how serious the pollution problem was.
(A) they realized
(B) did they realize
(C) had realized
(D) have they realized
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統計: A(4), B(40), C(4), D(8), E(0) #3493855
統計: A(4), B(40), C(4), D(8), E(0) #3493855