if the economy continues to slow引导条件状语从句。

which could stimulate activity but would add to引导定语从句，修饰purchase of debt or mortgage securities。

2.France has angrily dismissed a United Nations report which called for an end to what it described as the collective deportation of Roma people.

参考翻译：

联合国一份报告要求法国停止所谓的对罗马人的集中驱逐。法国强烈谴责这份报告。

结构分析：

which called for是定语从句，修饰report。what it described as是what引导的名词性从句，是called for an end to的宾语。

3.Police in Mexico say they are deeply concerned for the safety of a senior detective who's been leading an investigation into the mass killing of foreign migrants, whose bodies were found at a ranch in the north of the country on Tuesday.

参考翻译：

周二，警方在墨西哥北部一个农场发现大量外国移民被杀。墨西哥警方表示，他们非常担忧最这件事情进行调查的高级探员的安全。

结构分析：

这句话的主干是：Police in Mexico say they are deeply concerned for the safety...

who's been leading an investigation into是定语从句，修饰senior detective。whose bodies were found也是定语从句，修饰foreign migrants。