重点讲解：
1. crack down on 严厉打击；镇压；
There's going to be a nationwide drive to crack down on crime.
将有一场打击犯罪的全国性运动。
2. gang up 结伙，拉帮结派，联合起来（对抗某人）；
Several companies gang up to raise prices.
几家公司串通一气抬高物价。
3. touch upon 谈及；涉及；
She really could not touch upon such an indelicate subject.
她实在不想触碰如此令人窘迫的话题。
4. step down 辞职；下台；让位；
He has decided to step down as director of finance.
他决定辞去财务经理的职务。