Nowadays scientists are working hard to find out ______ to do with air pollution.

A．how

B．why

C．when

D．what

下述金属烤瓷冠基底蜡型应具备的要求

A．表面形态无尖锐棱角、锐边

B．蜡型的厚度至少应在0.8mm以上

C．蜡型厚薄均匀

D．金瓷交界处呈光滑凹面

E．颈缘与基牙密合