Sentence Correction

[BTGmoderatorRO]

Clark and Florence Wallace, a husband-and-wife medical team, worked steadily and efficiently through the night, but sipping their coffee the next morning, she noticed that he seemed disoriented.
(A) sipping their coffee the next morning, she noticed that he seemed disoriented
(B) sipping their coffee the next morning, he seemed to be disoriented, she noticed
(C) as they sipped their coffee the next morning, she noticed that he seemed disoriented
(D) as they were sipping their coffee the next morning, he seemed, she noticed, disoriented
(E) he seemed disoriented, she noticed, sipping their coffee the next morning

OA is C

This question is looking tricky, I thought D should be the right answer but my thoughts are drifting.