求助一道数学PS题??

banglixing

OG12 PS 112:
An operation θ is defined by the equation
a θ b =(a – b) / (a+b), for all numbers a and b such that a ≠ –b. If a ≠ –c and a θ c = 0, then c =
(A) –a
(B) –1/a
(C) 0
(D)1/a
(E) a

The answer is E.
OG answer explanation: Substitute c for b and 0 for a θ c in the given equation, and solve for c.

Why can we substitute c for b???

Thanks guys