A student measures the acceleration of free fall by means of a small metal ball, as illustrated. When switch $S$ is turned on, the ball is released from an electromagnet and an electronic timer begins. The ball then drops straight down. The timer comes to a stop when the ball lands on a trapdoor. The student measures the distance $PQ$ between the electromagnet and the trapdoor. This distance, together with the timer reading, is then used to calculate the acceleration of free fall. Which statement about errors in the experiment is correct?
- AThe random error can be reduced by adding the diameter of the ball to the distance $PQ$.
- BThe random error can be reduced by subtracting the diameter of the ball from the distance $PQ$.
- CThe systematic error can be reduced by adding the diameter of the ball to the distance $PQ$.
- DThe systematic error can be reduced by subtracting the diameter of the ball from the distance $PQ$.