The diagram illustrates a leaf on a plant that is being used in a photosynthesis experiment. At the beginning of the experiment, the leaf contains no starch. A piece of black paper is fixed to the upper surface of the leaf. The plant is kept in bright light for $12$ hours. Four discs are then removed from the leaf at the positions indicated on the diagram. The discs are tested for starch. Which disc has starch?
- Adisc A
- Bdisc B
- Cdisc C
- Ddisc D