（2）so as to
“so as to” is an interesting construction. i don't think that gmac likes it very much -- i've never seen it in an officially correct answer -- but i think the main distinction is that it's used to describe indirect intentions.【so as to在GMAT中從來沒有成為過正確選項。so as to表達的是indirect的目的】
Joe bought a gym membership so as to become more attractive to women.
Obviously, the act of buying a gym membership itself is not going to make joe instantly more attractive to women (women do not swoon over a man's gym membership card). Instead, there are plenty of implied intermediate steps between “by a gym membership” and “be more attractive”; the use of “so as to” implies that there are such intermediate steps in the process.
by Ron