Even Though the Autumn May Be Unusually Warm,the Leaves from Deciduous

Even though the autumn may be unusually warm,the leaves from deciduous trees begin to drop during September.Which of the following is the best explanation?  

A) The trees are responding to shorter days.
B) The autumn months are usually wetter because the summer rains were delayed.
C) The effects of phototropism are operating "right on schedule."
D) There is not enough sunlight to carry on photosynthesis any longer.
E) The trees are responding to consistent temperatures.