The students did poorly on the test more because they had not studied than the material was difficult.
The answer is:
The students did poorly on the test more because they had not studied than because the material was difficult.
Shouldn't it read as:
The students did poorly on the test more because they had not studied than because they didn't understand the material was difficult.
They had not studied is a clause and usually don't parallel clauses need to begin with the same word?
Thanks!