A) Speed is a scalar quantity because it only has magnitude (how fast an object is moving) and no direction.

Distance $= 160 \text{ m}$
Speed $= 16 \text{ m/s}$

The time it takes the horse to run a distance of $160 \text{ m}$ is $\boxed{\text{10 s}}$.

e) In the last $20 \text{ s}$, the horse maintains the same speed of $16 \text{ m/s}$. This means its velocity is constant, and therefore its acceleration is zero. According to Newton's First Law of Motion, if an object's acceleration is zero, the net force acting on it is also zero.

f) For the last $20 \text{ s}$, the horse maintains a constant speed. In a distance-time graph, constant speed is represented by a straight line with a positive gradient. Since the problem asks for a sketch for the last 20 s, we assume this is the period after it has reached its constant speed. The graph would show a linear increase in distance over time.

g) In the first $10 \text{ s}$ of Fig. 1.2, the speed of the horse increases from $0 \text{ m/s}$ to $16 \text{ m/s}$. The curve shows that the gradient is decreasing, meaning the acceleration is decreasing over this period. The horse is accelerating, but its acceleration is not constant; it is getting smaller as time progresses.