Because they didn't write their answers in complete sentences.
Fragment.
Working on the homework for 2 hours.
Fragment.
Taking summer quarter off seems like a good idea.
Complete sentence.
So many different things to choose from.
Fragment.
There were wonderful veiws from each room of the hotel.
Complete sentence.
She redid her assignment and got a better grade.
Complete sentence.
Because I want to write an essay, I am studying English.
Complete sentence.
During summer, when there are fewer classes.
Fragment.
Although he knows English very well.
Fragment.
One in a while, but only when the weather is good.
Fragment.
Zoom class was great, but they missed being in the classroom together.
Complete sentence.