Elias is a poor black worker (Mandela提供过指导)on the legal problems.(whom)to whom Mandela off

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(答案→)to whom Mandela offered guidance 
解析：考查定语从句，先行词是a poor black worker，动词词组：offer guidance to意思为向某人提供指导，将介词to提到关系代词whom前面，引导定语从句，也可以说whom Mandela offered guidance to。句意：Elias是Mandela提供过法律指导的一个贫穷的黑人。