For a particular function f(x), if x is a positive integer, then f(x) is also a positive integer. Furthermore, for any positive integer x, if f(x) is a multiple of 3, then f(x) + 1 is a prime number.
If f(9) = 18, which of the following must be true?
f(18) is a multiple of 3.
19 is a prime number.
f(10) is a prime number.
f(x) is always an even number.
