I think it is C, because if I'm right present principal is something that is happening presently? If so, C would be the answer because they "are doing" the exercises right now in context.
Can you be a little more specific? Hope i can help..
Phones should not be used in class because they are a major distraction. If a phone goes off in class not only does it distract the owner of the phone, but everyone in class instantly looks at the person whose phone went off. Which also brings up the problem with social media, and texting. Oxford learning states that "Students check their phones in the classroom an average of more than 11 times a day. That can add up to a lot of time spent distracted from schoolwork." But with that problem its also a sign of immaturity, In my opinion if they are gonna be on their phone anyways just let them. Because there is always that one kid that if you say No they will do it anyways. I think a solution for most the distraction is to say "Everyone turn your phones on silent, If I hear it go off its mine." That stops the hole surrounding distraction.
