(A) Since a serious health hazard
(since conj 因為,自從)--語意不通
(B) It was once a serious health hazard, measles can now be prevented by a vaccine. --兩個動詞文法不通
(C) That once a serious health hazard +動詞
(D) Once a serious health hazard
(Once conj 一旦 )
19. _____, measles can now be prevented ..-阿摩線上測驗