The cost of text books for school increases with the average class size identify the independent and dependent quantity in the situation.
a. cost; average class size
b. cost; number of class
c. average class size; cost
d. number of classes; average class size

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.