A"brain drain" has occurred in many sub-saharan african countries because
a. cyclical migration causes people to move between rural and urban areas
b. farmers move to new areas to find more productive land
c. professionals migrate to other continents that offer better opportunities
d. the natural fertility rate in the region is high

The best answer out of these four is letter C.

Amath expression is a collection of one or more terms, which are separated by the signs ( + , - , x , / ).example :   8+b8+b  or  a−3a−3in algebra, an equation is a mathematical statement with two expressions where, each expression is separated by an equal (=) sign. variables can be present on both the sides of the equality (=) sign and we use letters to denote such variables.example 1:   x=4+2x=4+2  , [here, we can observe that the variable is present only on one side of the equation].example 2:   2x+3=x+42x+3=x+4  , [here, we can observe that the variables are present on both side of the equation].