Terrorism(恐怖活动) has been the biggest__in the world.是填matter还是problem?

问题描述:

Terrorism(恐怖活动) has been the biggest__in the world.是填matter还是problem?

problem牛津词典对它的解释是thing that is difficult to deal with or understand 难以处理的事情; 难以了解的事物; 难题:How do you cope with the problem of poor vision?怎样解决视力不好这一难题呢?...