List three factors that have contributed to world population growth in the 20th century:

Increased Food production, Better medical Facilities, High Birth and Low death rate.

Increased food production has helped people in drought stricken areas and other people who previously had less access to food to have more. Better medical facilities are helping people cue and control the diseases. Because of the medical facilities birth rate is getting high as compared to death rate which is also contributing to growth in world population. Another reason according to some is the mass immigration of people from developing countries to developed countries where they get better lifestyle and better survival.  

b) to encourage free trade