Because C & ltG=AB

So the existence of a belongs to a.

B belongs to B.

Such that x=ab

Then b = a (- 1) x

And a (- 1) belongs to a and is contained in C.

So b belongs to C.

So b belongs to b and C.

Then x belongs to A(B passes through c)

Because a is contained in C.

B passes through c and is contained in C.

So A(B passes through c) is included in c.

To sum up, there is C=A(B to c)