我想你会做：

C(a) references A(a)
C(b1, c) references B(b, c)
C(b2, c) references B(b, c)

不过，这是一个有点很难说没有数据的例子，知道哪些列和表的意思。

But it is a little hard to tell without examples of the data and knowing what the columns and tables mean.