What Did Darwin's Experiments on Phototropism Illustrate

What did Darwin's experiments on phototropism illustrate?

A) that signals from plant roots control phototropism
B) that a translucent cap placed over a shoot tip will cause a plant to both grow and bend toward light
C) that a translucent cap placed over a shoot tip will cause a plant to bend toward light, but will inhibit growth
D) that an opaque cap placed over a shoot tip will cause a plant to bend toward light, but will inhibit growth