By subtracting both number you will be left off with the distance between the numbers.
Edit: answer (C) is wrong because it refers to the absolute values of p and q, the absolute value of a number is equal to how many numbers that number is away from zero.
Subtracting one number from the other will igve you the difference between them. However as P and Q are not specific numbers we do not know which is larger. Therefore we must use absolute value to make sure we do not end up with a negative number by subtracting the larger number from the smaller one.
