We know from the experiments of the past that plants bend toward ...

[xclash]

We know from the experiments of the past that plants bend toward light because
A) they need sunlight energy for photosynthesis.
B) the sun stimulates stem growth.
C) cell expansion is greater on the dark side of the stem.
D) auxin is inactive on the dark side of the stem.
E) phytochrome stimulates florigen production

[clippers!]

I chose C and got the answer, hope that helps.