A Dog Is Standing in the Tail Bed of a Pickup

A dog is standing in the tail bed of a pickup truck. The tail bed is coated with ice, causing the force of friction between the dog and the truck to be zero. The truck is initially at rest, and then accelerates to the right, moving along a flat road. As seen from a stationary observer (watching the truck move to the right) , the dog

A) does not move left or right, but simply slides toward the back of the truck.
B) moves to the right, but not as quickly as the truck is moving to the right, causing it to slide toward the back of the truck.
C) moves to the right at the same rate as the truck, so it doesn't slide.
D) moves to the left, as the truck moves to the right, causing the dog to slide toward the back of the truck.