Don't really worry about pushing too hard, it's even ok if you were to break a rib or two. it's better to push too hard than not hard enough
A person who smokes 1.5 packes daily gets 300 hits of nicotine to the brain each day. One cigarette has an average 10 nicotine in it. If one and a half pack has 30 sticks of cigarette. Nicotine is very very dangerous to a man's health. IT causes several types of caner. But the answer to your question is letter C - 300.
Do they work you mean?? Different variations of the position of your hands hits different muscle parts. Like doing push-ups with your hands far apart hits more of the chest area. Doing them with your hands close hits more of your triceps. I personally like them close.