Answer:
Muscle size and training specifiicty
Explanation:
It depends on all other factors such as anatomy and psychology, muscle fiber recruitment, motor pattern development and muscle memory but to increase modifiable strength, size plays no big role.
1. all of the above
2. hydrogen and oxygen
3. not 100% shure but a seems most right
4. true
The force the box is exerting on Manuel is the weight of the box, downward:

and this force is perfectly balanced by the constraint reaction applied by Manuel's hand, pushing upward.