重点讲解：
1. add to
添加；增加；掺加；
eg. A few jokes add leaven to a boring speech.
几句笑话可给枯燥无味的演讲增添活跃的气氛。
eg. Her comments will add to the uncertainty of the situation.
她的批评将会使局势更加不稳定。
2. be confined to
受限制的；被限定；
eg. The problem is not confined to Germany.
问题不仅限于德国。
eg. These dangers are not confined to smokers.
这些危险不只是困扰吸烟者。
3. threnten to do sth.
（坏事）可能发生，将要发生；
eg. Such measures threaten to provoke escalating retaliation and a full-blown trade war.
这些措施将招致更大规模的贸易报复措施，甚至全面的贸易战。
eg. He can even threaten to call the deal off.
他甚至还可以威胁要取消这笔交易。