Charles and Francis Darwin observed that covering the tips of canary grass seedlings prevented them from bending toward the light at the growing region a few millimeters below the tip. This result is best explained by the role of auxin in blocking the phototropic response.

a. the cessation of polar auxin transport.
b. positive charge of the auxin molecule.
c. negative charge of the auxin molecule.
d. transport of auxin from the plant tip to the elongation region.