You are correct in assuming that the answer is A. He is being thorough in running more tests.
I'm not going to do 3-5 sentences but practicing speaking clearly is necessary for a group discussion to be successful because when you speak clear everyone can understand you and you if you don't like speaking in front of a class or anything it will stop you from having to say it again. I know this wasn't long but I hope it helps some!!
It's D because you're comparing Chicago's tallest building to any other building in the city
