刘振民E2-5求助

Several recent studies suggest that a child born into a family whose members have allergies will probably themselves develop allergies following the onset of a minor viral infection.

(B) children born into families whose members have allergies will pobably themselves deveop allergies

(C)a child born into a family the members of which have allergies will probably develop an allergy.

答案是B, 我的疑问在于划线部分的following the onset of a minorviral infection, 是一个单数, 那么前面用a child...引导的宾语从句应该更合适吧?

lzm给的解释是C的of which have allergies是一个非限制性定语从句,应该用两个逗号引起来. 我查了语法书对于非限制性定语从句的定义, "非限制性定语从句是先行词的附加说明，去掉了也不会影响主句". 而这句话里的这个定语是原文必不可少的信息, 因此应该是个限制性定语从句才是, 也就是C这样没有错

应该怎么理解? 谢谢!