Guard cells are the only cells in the epidermis that contain chloroplasts and can undergo photosynthesis. This is important because

A) chloroplasts sense when light is available so that guard cells will open.
B) photosynthesis provides the energy necessary for contractile proteins to flex and open the guard cells.
C) guard cells will produce the O₂ necessary to power active transport.
D) ATP is required to power proton pumps in the guard cell membranes.
E) chloroplasts sense when light is available so that guard cells will open and guard cells will produce the O₂ necessary to power active transport.