Why do two objects have the smallest distance when their speeds are equal?
B is accelerating at a constant speed, so when the speed of B is less than that of A, A will keep approaching to B at a faster speed, so the distance between them will keep shortening until the two speeds are equal, and the distance will not be shortened (it is relatively static at this time), and then B will accelerate, so it will widen the distance from A again, so the distance is the smallest when the two speeds are equal.