Review the item:
During the exam, Mary sucked the ends of her hair,
chewed the top of her pencil, and
all the while she was picking at a scab on her
elbow, but she could not remember the third cause of the French Revolution.
When you list actions, you must use parallel structure. Here, sucked and chewed are both simple past tense verbs, but all the while she was picking at a scab on her elbow is a main clause, wrecking the parallelism.
To fix the item, change all the while she was picking to picked.