In case of plant breeding,cross hybridization is a time-consuming and tedious process because:

• A
  Pre-existing genetic variability is collected from wild varieties,species,and relatives of the cultivated crop species.
• B
  It involves the selection of plants among the progeny of the hybrids with the desired combination of characters.
• C
  It involves emasculation and bagging techniques to transfer desired pollen grains to a desired plant.
• D
  Both $(a)$ and $(b)$.