(a) must be true. (b) not necessarily true. (c) not necessarily true.
Work Step by Step
(a) Their velocity vectors must point in the same direction since they move on the same path. (b) Their velocity vectors may not have the same length. The length measure the speed of the objects, which may not be the same. (c) Their acceleration vectors may point in different directions, since the rate at which their speed changes may be different for the two objects.