重点讲解：
1. account for （数量或比例上）占；
The minority nationalities account for six per cent of the population.
少数民族占人口的百分之六。
2. put forward 提出（计划、建议）；推荐；
He answered up to all the questions put forward by the lawyer.
他很快应答了律师提出的所有问题。
3. serve as 用作；可当…使用；充作；
Water may serve as a type of instability.
水可以作为不稳定性的象征。
4. be shrouded in 隐瞒；保密；掩饰；掩盖；
The city was shrouded in thick smog.
烟雾笼罩着那座城市。