此题为定语从句语法中,介词+关系代词类型题.这类题很哟规律,即,先行词或者介词后的名词指代人,就用whom,指代物,就用which.
根据题中空前的定语从句的先行词relatives指代人,或者认为from后面应该是relatives,确定使用whom.
假如这句话，你选择them,整个句子就无法联系为整体.因为代词them不具备连接句子的功能. 比较下面的句子:
They arrived at a farmhouse, in front of which sat a small boy.
= They arrived at a farmhouse, and in front of it sat a small boy.
但是不可以说They arrived at a farmhouse, in front of it sat a small boy.
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