1. Why do Watergate culprits keep collective silence?
    为什么水门事件的罪犯们全都保持沉默?

2. However, the collective smoke from thousands of factories, over the past two centuries, has caused enormous increases in air pollution levels worldwide.
    但是，在过去的两个世纪中，从千万家工厂排放出的烟尘已大大加剧了全球大气的污染度。