This is not because a person arrives his natural limit of efficiency when he reaches the sixth grade,for it has been shown again and again that with special tuition so many older children,and also adults,can make enormous improvement.
arrive 永远是“不及物动词”,不可能直接跟宾语,如果跟宾语,需要加介词in 或 at(尤其表示抽象意义)。请网友核查原文,是不是漏掉了at?
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