Which of the following statements describes the absolute value of a number "a"?

a) the opposite value of a number "a" can be represented by the absolute value of a number "a."
b) given |a|, "a" is always positive
c) the distance from the number "a" to 0 on a number line can be represented by the absolute value of a number "a."
d) the positive value of a negative number "a" can be represented by the absolute value of a number "a."