_____ private cars bring us convenience,  they also cause more traffic accidents and pollution.

 ________ nearly all behavior is learned behavior is a basic assumption ________ has been put forward by social scientists.

All the students were studying in class　　　 Tom;he was playing football on the playground.

 _____ to go abroad, his spare time is totally spent in learning English.

Steps must be taken _____ this kind of disaster will never happen again.