So that x So as to: Qual utilizar

Hey, it's my first question here. I'd like to know how I can use so that and so as to. I was doing some exercises and then appeared a question that left me with a doubt:

He turned the stereo down very low _ he shouldn’t disturb the old lady downstairs.
A) so as to
B) in order to
C) nevertheless
D) even if
E) so that

Answer key:

e

What could I use in this case? I also will ask why couldn't I use in order to?
Please if I made any mistake in this topic/text let me know.
Thanks in advance!