They had learned six new lessons ___ the end of last week.

_____ this something, still unseen, they gave the name radium. 

My friend Tom stays at Internet bars above 8 hours every day ______, really a netter(网虫).

 I’ll be back _____ 5 o’clock tomorrow afternoon.

_________ the fact that many drivers are ever trained, we cannot stress too much theimportance of traffic safety.