In AMNO, the measure of 20=90°, the measure of ZN=70°, and OM = 3.3
feet. Find the length of MN to the nearest tenth of a foot.

a is the correct

step-by-step explanation:

points a and b are equidistant from the desired line, so the line you find by using them will go through point r.

using points other than a and b will give the desired line only if they are equidistant from r. if they are randomly located, the perpendicular line you create will not necessarily go through point r, hence will not be a solution to the problem.