Why Was the Kepler Mission Not Able to Find Planets

Why was the Kepler mission not able to find planets smaller than Mars, even though it was in space (and had no Earth atmosphere to deal with) ?

A) Such planets always take longer to orbit their stars than the time the mission lasted
B) Such planets make dips in the light of the star that are too small for Kepler to detect
C) Such planets are only ever detectable using the Doppler shift method
D) Such planets are red in color, and Kepler's cameras could not see red objects
E) Astronomers believe that planets smaller than Mars could not exist