All of the following proportions are equivalent except a/b=c/d b/a=d/c a/d=c/b a/c=b/d

I believe the correct answer from the choices listed above is the third option. All of the following proportions are equivalent except a/d=c/b. 

a/b=c/d

b/a=d/c => a/b = c/d

a/d=c/b not equal to a/b = c/d

a/c=b/d => a/b = c/d