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.

Quick! a survey of 57 customers was taken at a bookstore regarding the types of books purchased. the survey found that 33 customers purchased mysteries, 25 purchased science fiction, 18 purchased romance novels, 12 purchased mysteries and science fiction, 9 purchased mysteries and romance novels, 6 purchased science fiction and romance novels, and 2 purchased all three types of books. a) how many of the customers surveyed purchased only mysteries? b) how many purchased mysteries and science fiction, but not romance novels? c) how many purchased mysteries or science fiction? d) how many purchased mysteries or science fiction, but not romance novels? e) how many purchased exactly two types of books?