President Obama told television viewers that he saw as his duty to run for a second term.

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A. one
B. that
C. this
(答案→)D. it
解析：考察it作为形式宾语。本句指的it是形式宾语，真正的宾语是后面的不定式to run for a second term。在英语中只有it可以作为形式主语或形式宾语。句意：奥巴马总统告诉电视观众他把做第二任总统当成是自己的职责。故D正确。