The sentence "____ if and only if $ x + x = 3x$" is TRUE. Which of the following could be used to fill in the blank?

Question

$\begin {array} {1 1} (A)\;x + x = 2x & \quad (B)\;2x - x = 2x \\ (C)\;both\: 1 \: and \: 2 \: could\: be\: used & \quad (D)\;neither\: 1\: nor\: 2\: could \: be\: used \end {array}$