What countries were the most heavily populated countries in the 20th century?

the topic i chose to write about was " why scuba diving is so popular". the topic is suitable for the audience because it is the easiest to develop and explain, it can be explained in a simpler more conversational manner, it does not require the use of too many technical words, and the final reason for selecting this topic is that it is the one most people can relate to, you might not know too much about scuba diving, but you could probably agree that it is very popular. the purpose of the writing about this topic is to convince more people about the benefits both for the body and the mind that   scuba diving can have on a person. it can you keep fit since it requires a lot of energy when swimming underwater; it also you relax because you forget about the world when you are underwater being mesmerized by the underwater world. i would write this paper in the summer, a perfect time to give scuba diving a try if a person has not done it yet.