Which car is accelerating?

Responses
A a car that is unmovinga car that is unmoving
B a car that rounds a curve at a constant speeda car that rounds a curve at a constant speed
C a car that travels in straight line at a constant speeda car that travels in straight line at a constant speed
D a car that is set to a constant speed of 60 miles per hour

The correct response is D - a car that is set to a constant speed of 60 miles per hour.