. — I sent him the dictionary by mail this morning .

—You ____ that; he''s coming tomorrow afternoon .

A．needn''t have done

B．didn''t have to

C．needn''t do

D．mustn''t do

氧化镁、三氧化硫，是水泥中的有害成分，如果含量超标，将会形成膨胀性破坏，是导致水泥不安定的主要因素。