target is right. "so as to" = in order to, NOT "so X as to"
I think the explanation in the explanation in 33 is incomplete. Here's the question:
Although...
it is so debilitating that it has become an economic drain...
C: so debilitating is it as to become an economic...
when the answer key says "so X as to Y" is not idiomatic, I think it's referring to this case which is "it is so
debilitating is it as to..."
if the sentence had read: "it is
so debilitating as to become an economic drain...", the sentence would be acceptable.
EDIT:
stacey (not sure if it's the same person) has a short discussion here:
https://www.manhattangmat.com/forums/x-i ... t6537.html
after some search, I think "so X as to (be) Y" is correct as long as the
meaning is correct. Also, if one of the choices is "so X that...", pay attention because usually "so... that..." is preferred.