The runners have just started a race. They started at rest and are getting faster. What statement best describes the motion of the runners at the beginning of the race?
A.
Acceleration is positive and speed is increasing.
B.
Acceleration and speed both remain constant.
C.
Velocity and speed both remain constant.
D.
Velocity remains the same and speed is positive.

A.  Acceleration is positive and speed is increasing.

The motion performed by a runner is part of a uniformly accelerated motion with constant acceleration.

Equation used  

vf = final velocity  

vi = vo = initial velocity  

When a runner moves from rest, his speed will increase from being at rest (vi=0) and then move with a constant acceleration(a), and as soon as he crosses the finish line his acceleration will decrease (experience a deceleration ⇒a=- ) and decrease his speed until it finally stops

Velocity and speed are different because velocity itself is a vector quantity that takes into account direction and displacement, while speed is a scalar quantity (only distance).

So the speed does not take positive or negative values because it involves direction, while acceleration takes positive and negative values (deceleration) because it takes direction.