The mean of a set of four test scores is 79. if three test scores are 77, 79, and 85, what is the fourth test score?
a. 83
b. 81
c. 79
d. 75

Set $r$ is a set of rectangles such that (1) only the grid points shown here are used as vertices, (2) all sides are vertical or horizontal and (3) no two rectangles in the set are congruent. if $r$ contains the maximum possible number of rectangles given these conditions, what fraction of the rectangles in set $r$ are squares? express your answer as a common fraction.