The question asks for the following: "In three-dimensional space, if each of the two lines L1 and L2 is perpendicular to line L3, which of the following must be true?"

(I.) L1 is parallel to L2
(II.) L1 is perpendicular to L2
(III.) L1 and L2 lie on the same place

The correct answer is E "none of the above". I chose a different answer at the start, because a few of these statements appeared very plausible. As suggested in the solution, am I supposed to always look for a statement that is sufficient to be true for these types of questions?

Hey Atomico,

Actually, when a question asks "which MUST BE TRUE" your job is to find a way to eliminate the statement by showing that it "COULD BE FALSE". The fact that a statement is "plausible" or even "probable" doesn't help...you have to prove that it's 100% certain in order for it to be correct, and often the best way to go about it is by finding that case in which it is not necessarily true so that you can eliminate it.