obamaspeech

这是考察定语从句的用法。 many of whom的whom指的是前面的youths。

如果要选A them的话，那就是并列句，应该改成 .

Barack Obama delivered a speech to 500 local youths during his visit to China and many of them were from Fudan University and Tongji University.

请注意，many前面是逗号，不是and，所以不是并列句，而是定语从句。