The World Health Organization wishes to estimate the mean density of people per square kilometer, they collect data on 56 countries. Justify for full credit. 

1. (a) Which of the followings is the variable? 

1. (i) All countries in the world 
2. (ii) Density of people per square kilometer 
3. (iii) Set of densities of people per square kilometer of all countries 
4. (iv) Set of densities of people per square kilometer of 56 countries