Why Does a Genomic Library Need More Clones Than What

Why does a genomic library need more clones than what would be estimated by calculations to be complete?

A) There is a significant probability that some sequences would not be represented in a library if it was only the calculated size
B) The restrriction enzyme that is used to prepare the DNA for cloning will not digest certain sequences within the DNA.
C) The restriction enzyme used to prepare the DNA for cloning sometimes leaves a different sticky end
D) Restriction enzymes cleave DNA preferentially at the ends of open reading frames so coding regions are prefereentially cloned.