Wemust______thattheexperimentiscontrolledasrigidlyaspossible.

Each year in the United States, many black teenagers _______ of school, either because they cannot keep up or they have to work to support their family.

________,Johnhasbeentoseveralforeigncountries.

TheTVstation,in________tomassivepopulardemand,decidedtocontinuethesoapopera(肥皂剧).

The problem ________ when the students refused to do their homework.