Answer:
by combining sentences 4 and 5 with the transition word so
Step-by-step explanation:
The transitional word so indicates that something occurs as a result of another thing. Thus, the fact that students are concentrated on their tasks and enjoying themselves is a reason why discipline problems are reduced. Besides, it is suitable to join sentences four and five into a more complex sentence; otherwise, there is a lack of parallelism and connection between the two sentences.