QUESTION 21 The northern lights are bright flashes of colored light between 50 and 200 miles above Earth. Aflash occurs 100 miles above Earth at point C. Find the measure of
BD the portion of the Earth from which the flash is visible. Round answer to the nearest degree,

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.

answer:

c 60% on edg