The Optimum Angle of Tilt (From the Horizontal) for a Flat

The optimum angle of tilt (from the horizontal) for a flat plate collector at 35 $\degree$ N latitude for the purpose of space heating is ________.

A) 0 $\degree$
B) 25 $\degree$
C) 35 $\degree$
D) 45 $\degree$
E) 75 $\degree$