Which one of the following is not a correct explanation of cross-pollination?

• A
  The pollen grains are transferred from one flower to another flower situated on the same plant.
• B
  The pollen grains are transferred from one flower to another flower,of another plant of the same species.
• C
  Both $A$ and $B$.
• D
  The pollen grains of the flower are transferred to the stigma of the same flower.