(2006? Binzhou) As shown in the figure, if the main vein of one leaf of a plant is cut off (A in the figure), opaque tin foil paper is pasted on both sides of the other leaf (B in the figure).

The purpose of this experiment is to exhaust the original starch in leaves after 24 hours in the dark. The variables of B leaves, shady parts (both sides are covered with opaque tin foil paper) and non-shady parts are light, and it can be concluded that light is the condition of photosynthesis. C leaves are shaded without photosynthesis, and the iodine solution does not turn blue.

The variable of nail leaves a and b is water. After the main vein is cut off, the conduit at the cut-off place is cut off, resulting in that water can't be transported to leaf A. Photosynthesis at leaf A stops due to lack of raw materials, and starch can't be produced, so the drop of iodine doesn't turn blue. Below the cut-off place, water transported by the conduit can be obtained for photosynthesis, so starch can be produced at leaf A, and the drop of iodine turns blue.

So choose: C.