C
since the quantities vary directly then
x = kf(x) ( where k is the constant of variation )
to find k use the given condition , f(x) = 40 when x = 2
k = [tex]\frac{x}{f(x)}[/tex] = [tex]\frac{8}{40}[/tex] = [tex]\frac{1}{5}[/tex]
thus x = [tex]\frac{1}{5}[/tex]f(x)
when x = 2
f(x) = [tex]\frac{x}{k}[/tex] = [tex]\frac{2}{\frac{1}{5} }[/tex] = 10
