Never before had the editors faced so many challenges at once as they had in compiling the 22nd annual compendium of the Archeological society of North America.

A. so many challenges at once as they had in
B. at once as many challenges as
C. at once as many challenges that they were when
D. as many challenges at once as they confronted in
E. so many challenges at once that confronted them in

OA: D

What's wrong with options B and C?