I think is very important for students to study by themselves in the school or at home.
A. it B. this C. that
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解析：考查代词it 做的是形式主语，真正的主语是后面的不定式。此题中I think后是一个宾语从句，it做主语，真正的主语为to study by themselves in the school or at home，句意：我认为对学生们来说在学校或在家里靠自己学习是非常重要的。故选A。