It causes breathing problems, cancers, organ damage, changes your appearance for the worst, it smells bad, some of your friends may not want to be around you as much if they can't stand the smoke (like asthmatics), it can kill you, leaves a bad taste in your mouth, it costs a lot of money long term, and it is an addiction that is very hard to get rid of. The worst part is that you can also injure those around you in the same way with secondhand smoking.
Answer: 5(last one :regulation of circulatory system)
This is more of an opinion based question. We can't really help you. Sorry
All STDs have readily visible symptoms that are easy to detect.
