If it's only regarding the invention of a specific dental equipment, it'll be to inform. the student is preparing to share their knowledge on the dental drill.
to persuade is usually the purpose when there's a question or an argument being raised, and in this case, it doesn't sound like anyone would be against dental drills. dental drills is also a fairly dry topic, so it's doubtful that the student would be aiming to entertain. it isn't a demonstration because the student likely isn't giving a step by step how to in regards to using a dental drill.
The answer to your question is true
They could have a fear of thunder
I believe the answer is: sleep debt
Sleep debt refers to accumulative effect that keep increasing as people keep not getting enough time for proper sleep. Studies show that in the long run, sleep debt would resulted in decreasing overall productivity and the actual effect of repaying sleep debt could only be felt if the debt is paid within one week period.
Answer:
Speed of sound wave v=νλ=2000×0.35=700 m/s
Distance d=1.5 km=1500 m
Time t=vd=7001500≈2 s
