"F) Finally, we may define Pic(X) to be the group Div(X)/(pinc. divisors). Then R_X itself is a UFD if and only if Pic(X)=(0)."

This part is the 6th line from the top on the page 19 of "Algebraic geometry 1:Complex Projective Varieties(by David Mumford)"

Can you prove this?
Please, explain to me...