Sentence Correction

The ancient Romans piped hot water through the walls and under the floors so as to warm up the rooms.

A. so as to warm up the rooms
B. and so would be able to warm up the rooms
C. to warm the rooms up
D. so that they could warm up the rooms
E. in order that they would warm up the rooms

OA : D

refer Spideys notes.
So as is never correct in GMAT.
so that is the correct idion used here and hence D

Option A
looks wordy and it is not idiomatic. Thus this option is incorrect.

Option B.
awkward sentence not properly constructed.

Option C.
The expression in this option is not enough reason to back up the action performed by the Romans.
Thus, this option is incorrect.

Option D.
This is the correct answer as it clearly gives reason for the action performed and it a good construction manner

Option E.
'in Order' this is a wrong idiomatic expression in this option. Definitely, wrong.