Why is the surface of mercury covered with meteor impact craters, while earth's surface has relatively few creaters?

Which barrier to economic development is shown in the following examples? i. the high rate of hiv infection in underdeveloped nations ii. the use of children as soldiers in uganda’s army in the early 2000s iii. low literacy rates in underdeveloped nations with limited exposure to adequate education a. foreign debt b. political instability c. widespread disease d. low levels of social welfare