Critical Reasoning

The term "pit bull" does not designate a breed of dog, as do the terms "German shepherd" and "poodle." It is like the terms "Seeing-Eye dog" and "police dog," which designate dogs according to what they do. If you take two German shepherds and place them side by side, you cannot tell by appearance alone which is the police dog and which is the Seeing-Eye dog.

Which one of the following is the main point of the passage?

(A) German shepherds can be pit bulls.
(B) Pit bulls can be distinguished from other kinds of dogs by appearance alone.
(C) A dog is a pit bull because of what it does, not because of its breed.
(D) German shepherds can function both as police dogs and as Seeing-Eye dogs.
(E) Some breeds of dogs cannot be distinguished from other breeds of dogs by appearance alone.

Paraphrase:
Pit bull does not designate to a breed. It designates dogs according to what they do.
=> It indicates that a pit bull dog is so named not for its breed but for what is does. So, C is the answer.

My confusion:
If two German Shepherds are placed side by side and if we can not differentiate them which one is police and which one seeing, then can's we conclude that German shepherds can work both as police and seeing dogs?

Experts, please give some light on it.
Thanks.