The answer is D.
Factor production that could be changed during particular period is called Variable Factor of Productions.
Basically, factor of productions is divided to fixed and variable factor.
FIxed factors are the one that will always stay the same no matter how many products you made. For example electricity cost, building rent, etc
The variable factors differ depend on how product you produce, such as raw material.
Fast growing cities rarely had sewage systems which increased disease. Also many factories let out pollutants in the air. They also had few safety regulations and employed children, usually orphans, leading to many accidental deaths due to machinery and fires.
Preemption, is the answer you are looking for i believe.
