EachyearintheUnitedStates,manyblackteenagers_______ofschool,eitherbecausetheycannotkeepuportheyhavetoworktosupporttheirfamily.

When he suddenly ________ up the subject of genetic engineering, there was an embarrassedsilence.

Itisusefultoscience_______theearthasanobjectinspace.

The TV station, in ________ to massive popular demand, decided to continue the soap opera(肥皂剧 ).

It is useful to science _______ the earth as an object in space.