Independent variables are things that can stand alone. Which set of these variables does not rely on each other? For example the income and savings. I want 1,000,000 in my savings account, however my income is not nearly that high since I only make 19,000. Look at the other pairs and see if there is a correlation between them.
ANSWER
C.

EXPLANATION
The given function has range:
-3≤f(t)≤3
This implies that, the amplitude is

The function is symmetric with respect to the origin so this is a sine function.
This function is again reflected in the x-axis, hence
a=-3.
The period of this function is

This implies that, b=3,
hence the equation of the function is

F+s=95
f=s+27
s+27+s=95
2s=95-27
sixth graders = 34
fith graders = 61