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Read the following scenario to answer the following questions.
In the late 1800s, Charles Darwin and his son Francis conducted the first experiments on phototropism. Several years later, their work was furthered by Peter Boysen-Jensen, Arpad Paal, and Frits Went. Peter Boysen-Jensen separated the tip of grass shoots from the rest of the plant using either tiny blocks of agar (a gelatin) or a mica wafer (an impervious rock). Like the Darwins, Boysen-Jensen noticed that the grass did not grow toward a light without its tips. However, when he separated the tip from the rest of the plant using agar, the grass would grow toward a light. The grass would not grow toward a light if the tip of the shoot was separated using a mica wafer. Arpad Paal cut off the tips of grass shoots that were growing in the dark. He placed these cut tips back on the shoots, but with only part of the tip covering the cut surface, and found that the plants grew, in the dark, in the opposite direction from the side with the tip covering it. Lastly, Frits Went removed the tips of many grass shoots and placed them on a large block of agar for a few hours. Then, he cut up the agar block, and was able to make grass shoots without any tips at all grow toward a light by putting these agar blocks on the cut surfaces of the shoots (i.e., no tips were placed back on the shoots, only the agar).
Considering the experiments of the Darwins, Boysen-Jensen, Paal, and Went, what do you conclude about phototropism?
A) We still do not know the mechanism for phototropism.
B) The substance that regulates phototropism is produced in the tips of plants, and causes the same response even when it comes from an external source (not from the same individual).
C) For a plant to bend toward a light source, the substance that regulates phototropism must be able to move into cells on the same side as the light source.
D) The substance that regulates phototropism is distributed throughout a plant equally, but cells on the side away from the light source are the only ones that respond to the substance.

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