A Person Riding a Merry-Go-Round Passes Very Close to a Person

Multiple Choice

A person riding a merry-go-round passes very close to a person standing on the ground once (event $A$ ) and then again (event $B$ ) . Assume the ground is an inertial frame and that the rider moves at a constant speed.
-(b) Which person's watch measures the spacetime interval $\Delta s$ between those events?

A) The rider in the merry-go-round
B) The person standing on the ground
C) Both
D) Neither