Answer:
<em>C. the nation, and since then,</em>
Exclamation: If you're joining two sentences together you need a comma to do so. By placing a comma after "nation" and "then" you are making a run-on sentence.
Online learning has heavily affected students, due to the limited communication, meaning not all the students reach their full understanding of the subject. There are many negative effects to it, such as; the lack of efficiency of technology, the difficulty for pupils to understand the concepts taught, and online learning causes social isolation and results in pupils not developing the necessary communication skills.
