Which description applies to alternative mrna splicing?

RNA splicing is an important stage in gene transcription. mRNA transfers the information from DNA to proteins.  In many cases, this process creates multiple  proteins from the same gene by varying the exon composition. This is called alternative splicing and can happen because exons are extended or skipped, or introns can be retained instead of being removed.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.