Negotiators have made a breakthrough on the most difficult issue of employment security.谈判人员在最难解决的职业保障问题上已有了突破。The question of world hunger is a hard nut to crack.世界范围的饥荒是一个很难解决的问题。Drug abuse is one of the country's least tractable social problems.嗜毒是这个国家最难解决的社会问题之一。