Purebred dogs are first assigned the same litter number until they are individually registered with a name. Then the dogs will be assigned a unique registration number with a dog registry. Many people choose to have a microchip implanted in the dog so that it can be read to immediately find the dog's owner.

Which of the following relations is not a function?

A. (registration number, litter number)

B. (microchip number, registration number)

C. (litter number, microchip number)

D. (microchip number, litter number)