—The hall___ for international conferences is of great importance

—I see, and we are sure to complete it on time

A．built

B．to be built

C．having been built

D．building

下列各组物质按单质、化合物、混合物的顺序排列的一组是（    ）

A．红磷、干冰、石油

B．天燃气、水银、塑料

C．黄铜、汽水、碳酸钠

D．氦气、食醋、洁净的空气