The government officials met the workers and engineers working on the stadium, most were m

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A. of which
B. of who
(答案→)C. of whom
D. of them
解析：考查非限制性定语从句，先行词为the workers and engineers，他们中的大多数应用most of whom，句意：政府的官员在体育馆见到了许多工人和工程师，他们中的大多数是移民工人。所以选C。