Choose the biconditional statement with the same meaning as these two statements combined:
Statement 1: If a number is positive, then it is greater than zero.
Statement 2: If a number is greater than zero, then it is positive.
Select one:
a. A number is positive if and only if it is not zero.
b. If a number is positive and greater than zero, then it is not negative.
c. If a number is non-negative, then it is positive.
d. A number is positive if and only if it is greater than zero.