On a Clear Day, the Sky Appears to Be More

Multiple Choice

On a clear day, the sky appears to be more blue toward the zenith (overhead) than it does toward the horizon. This occurs because

A) the atmosphere is denser higher up than it is at the Earth's surface.
B) the temperature of the upper atmosphere is higher than it is at the Earth's surface.
C) the sunlight travels over a longer path at the horizon, resulting in more scattering.
D) none of the given answers