题目内容

— How many times have you been to America — It’s the second time I ______ there.

A. was
B. am
C. have been

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After supper my father read the newspaper, ______ my grandma listened to him carefully.

A. and
B. or
C. but

某生物碱的碱性强，则它的

A. ka大
B. Kb小
C. pKb大
D. pKa大
E. pKa小

I ______ my bedroom already. It is very clean now.

A. cleaned
B. have cleaned
C. clean

可将中药水提液中的亲水性成分萃取出来的溶剂是

A. 乙醚
B. 醋酸乙酯
C. 丙酮
D. 正丁醇
E. 乙醇