In pqr, point s lies on qr. if ps is perpendicular to qr, which term describes ps? a. perpendicular bisector
b. angle bisector
c. median
d. altitude

It can't be for sure a perpendicular bisector cuz you don't know what kind of triangle you are dealing with so that one's out.  It's not necessarily an angle bisector either for the same reason. A median means that PS will go through the opposite side right in the center, and again, depending upon what type of triangle you have, this could be true...So it is an altitude, which is like a height of a triangle. It drops down from a vertex and forms a right angle with wherever it goes through. So D.

C. expression because any expression or equation may contain more than one variable