A positive number A is shown on the number line as in the figure.
Then, on this number line; numbers whose distance from 0 is equal to half the distance of number A from 0 are marked.
If the distance from one of the marked numbers to number A is 6 units, what is the sum of the possible values of number A?
A) 15
B) 16
C) 18
D) 20
E) 21