Given Functions F and G, Perform the Indicated Operations $\mathrm { r }$

Given functions f and g, perform the indicated operations.
-The following graph shows the private, public and total national school enrollment for students for select years from 1970 through 2000 .
National School Enrollment

i) How is the graph for total school enrollment, T, determined from the graph of the private enrollment, $\mathrm { r }$ , and the public enrollment, u?
ii) During which 10 -year period did the total number of students enrolled increase the least?
iii) During which 10 -year period did the total number of students enrolled increase the most?

A) i) $\mathrm { T }$ is the sum of $\mathrm { r }$ and $\mathrm { u }$ .
ii) $1970 - 1980$
iii) $1990 - 2000$
B) i) $T$ is the sum of $r$ and $u .$
ii) $1990 - 2000$
iii) $1970 - 1980$
C) i) $T$ is the sum of $r$ and $u$ .
ii) $1970 - 1980$
iii) $1980 - 1990$
D) i) $\mathrm { T }$ is the difference of $\mathrm { r }$ and $\mathrm { u }$ .
ii) $1970 - 1980$
iii) $1990 - 2000$