Sandy is studying euglenas. She knows that euglenas use their eyespots to find light for photosynthesis. She hypothesizes that their eyespots will respond more positively to blue light than to red light. She places a group of euglenas in a container of water. She covers the middle of the container with black paper. She places a blue light at one end of the container and a red light at the other end. After 30 minutes, she finds more of the euglenas have gathered under the red light. Which characteristic of life are the euglenas demonstrating in this example