You May Have Noticed That a Plant Placed on a Windowsill

You may have noticed that a plant placed on a windowsill will eventually bend toward a light source. This is likely due to the fact that, in response to a light source:

A) plant cells will move from one side of a stem to the opposite side.
B) plant cells on one side of the stem will elongate.
C) all the cells of a plant's meristem will differentiate.
D) the cell walls of plant cells will break down.