1.Never before have we seen such severe food problem.
2.Undoubtfully,the accident was caused by the ignorance of public secure.
3.Petrolic burning leads to environment's pollution,therefore many countries think it's important to develop clean energy.
4.It's generous of you donating so much for quake victims.
5.If you don't konw how to pronounce this word,please consult teachers.
6.It wasn’t long before the coach made Tom substitute for the injured player.