1.C
1）两句之间并没有and等连词连接,说明后句是非限制性定语从句.
由此可以排除BD.如果要用them,则必须在第二句前加上and/but.
2）定语从句的关系代词which只能代指物,不能代指人；而whom刚好只能代指人,由此可以排除A.
3) 定语从句的关系代词 whom = 先行词 51 students,即完整的定语从句为：
none of whom failed in the test
= none of the 51 students failed in the test
2.D
此题考查部分倒装句型.
1）如果句子把only引导的状语/状语从句提前至句首,则该句/主句必须采用部分倒装,其变化如同变一般疑问句一样,把be/助动词/情态动词也提前.
所以该题主句they were able to leave变成were they able to leave
2) 而此时状语/状语从句则不能倒装,即此题的状语从句句序正常：when the match was over.
3）该句正常句序为：
They were able to leave only when the match was over.
4) 再如：Only in this way can you solve the problem.
上句正常句序为：You can solve the problem only in this way.