以下句子，强调的主题是研究者的活动的是。
（A）ThecausesofLidendriteformationinLi-metalbatterywereinvestigatedbyJones.
（B）TherehavebeenseveralinvestigationsintothecausesofLidendriteformationinLi-metalbattery.
（C）SomeresearcheshavestudiedthecausesofLidendriteformationinLi-metalbattery.
（D）ThecausesofLidendriteformationinLi-metalbatteryhavenbeenwidelyinvestigated.
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