Read the introduction from a biology book.

Think about the way your body feels after a swim in a cold pool. Think about the way your body feels after a hot bath. Temperature affects human beings in many ways. How does temperature affect plants? Do they photosynthesize faster in cold weather or hot? Students can measure the way sugar is transported within a plant at different temperatures to find these answers.

According to the introduction, why are plants being studied?

to see how much heat is produced by plants
to see if growth is improved in a cold pool
to see how temperature affects plants
to see if plants grow better with sugar water