isnecessarythatyoustarttrainingafewmonthsbeforet...

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is necessary that you start training a few months before the walk.

A. This B. That C. What D. It

D 考查it的用法。it可用作形式主语，构成固定句型“It is+adj.+that从句”，后面的that从句是真正的主语。故*为D。