-Where is our English teacher?-She is in the classroom, ________ the exercise ________ the students have done at home．A.explaining;/B.explaining; forC.explained; toD.explained; from
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A解析本题考查非谓语动词和定语从句引导词的用法。分析句子结构，第一空为现在分词作伴随状语，后面为定语从句，因引导词修饰先行词exercises，并在定语从句中作have done的宾语，所以可省略。