Variegated leaves of a plant were supplied with radioactive carbon dioxide $(^{14}CO_2)$ during an experiment. Leaf $X$ was kept in the dark and leaf $Y$ was kept in the light. At the end of the experiment,the radioactivity in the leaves was measured and found to be as shown in the diagram. What is the most likely explanation for the level of radioactivity found in the yellow zone of leaf $Y$?

• A
  Photosynthesis occurs but no storage of starch occurs in this zone.
• B
  Radioactive carbon dioxide diffuses into the leaf and accumulates here.
• C
  Products of photosynthesis diffuse into the yellow zone.
• D
  Photosynthesis proceeds slowly in the absence of chlorophyll $a$ and $b$.