求教in which 和where 的区别？

做了些语法题，虽然答案是选对了，但是看不太明白解答的意思，请教下大家~谢谢^-^

In the face of widespread concern about environmental waste, compact disc manufacturers are attempting to find a replacement for the disposable plastic box in which they package.（已代入正确选项）

（B）the disposable plastic box where they (解释中说，where在这里模糊，请问为什么说where模糊呢？语法中where不是可以等于in which的么？还是说GMAT中in which>where?)

另外还有道类似的是：

Several recent studies suggest that children born into families whose members have allergies will probably themselves develop allergies following the onset of a minor viral infection.(已代入正确选项)

（D）in those families where members have allergies, children will probably develop allergies themselves（解释中说，where在这里引导不准确，请问，为什么引导不准确呢?在这里where引导in those families不可以吗？）

以上2题都是来自与LZM的书上的..望NN们指教拉~谢谢