They explain that,

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选just,或者改为They explain that,somthing like electronic dictionaries can be useful on the study.
======以下答案可供参考======
供参考答案1:
选just, 因为they explain that后面明显接的是一个宾语从句，所以从句部分的主干应该是they can be useful on the study, just like electronic dictionaries是状语；如果你选something,认为they can be useful on the study是something的同位语从句，那么这个从句的主干就变成了something like electronic dictionaries,意思上是解释不通的，所以不对
供参考答案2:
B.something like electranic dictionaries
因为后面有同位语从句：
something=that they can be useful on the study.
所以A不行。