Some Types of Dinoflagellates (Single-Celled Eukaryotes) Are Photosynthetic

Some types of dinoflagellates (single-celled eukaryotes) are photosynthetic. They have flagella and can swim through the water. If you grow them in a container that is lit from the right side, the cells will swim to the right side of the container. This result suggests that dinoflagellates:

A) sense light.
B) sense Earth's magnetic field.
C) sense a heterotrophic food source.
D) swim in random directions.
E) swim in response to temperature gradients.