<p>In D, “it” could possibly refer to “effectiveness”, even though that may not “sound” like what the writer intends to convey. You need to eliminate all possible grammatical ambiguities.</p>
<p>For the method of plugging in numbers, generally speaking, you should avoid plugging in 0, 1, numbers in the original question , or equal numbers. In this case, plugging in 1 happens to reveal the correct answer.</p>