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 the same species

• C

  both $a$ and $b$

• D

  The pollen grains of the flower are transferred to the stigma of the same flower