科研之友
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摘要
<正>克里斯托弗很烦恼。最近的数学课越来越难了,他觉得自己已经跟不上同班同学了。Christopher sighed and glared at his math book with frustration(挫败). He usually had no trouble with math, but he did not understand what they were learning in class this week. Since beginning this new material,
