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

D and C looked fine to me. I picked C as I thought it's concise and almost conveys the indended meaning.
What are your views?