题目:
The top student failed again in the examination and only then ______ the importance of reading and memorizing the English texts.
A.he realized
B.he had realized
C.had he realized
D.did he realize
答案:
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答案:A①可以,澄清石灰水中通过量的CO2先生成碳酸钙沉淀,后溶解生成碳酸氢钙溶液; ②可以,向Fe(OH)3胶体中逐滴加入过量的稀硫酸先有红褐色沉淀生成,后沉淀溶解;③可以,向AgNO3溶液中逐滴加入过量氨水先...