单选题 15.  The text was too difficult for us, so our teacher tried to explain it sentence ______ sentence.
【正确答案】 C
【答案解析】[考点] 词语搭配
sentence by sentence为固定短语,意为“一句一句地,逐句地”。
   这篇课文对于我们来说太难了,因此我们的老师试图逐句解释。