When Skyler Was First Training His Dog, Smooches, to Heel

When Skyler was first training his dog, Smooches, to heel he would give Smooches a treat when she stayed close during walks. Now Smooches stays right by Skyler's side, even when she is not on her leash. Which of the following is true, for this situation?

A) Staying close was a negative reinforcer for receiving dog treats.
B) The dog treats were negative reinforcers for staying close.
C) Staying close was a positive reinforcer for receiving dog treats.
D) The dog treats were positive reinforcers for staying close.