Observers A and B move along the same straight track in the east–west direction. Observer B moves 4 m/s east relative to Observer A. Both observers independently measure the velocity of the same skateboard. Observer A measures the skateboard’s velocity as 3 m/s east relative to A, while Observer B measures it as 1 m/s west relative to B. Which of the following best describes the possible velocity of Observer A relative to the ground?
a) Exactly 4m/s east
b) Exactly 0m/s (Observer A stationary relative to the ground)
c) It may have any value; the observations are satisfied for every constant speed of Observer A.
d) Exactly 7m/s east