A Researcher Measures the Quantity of CDNA That Corresponds to the Exons

A researcher measures the quantity of cDNA that corresponds to the exons of a particular gene in mice using RNA- seq. They find that RNA for this gene isolated from the brain contains 4 exons, while RNA from the liver heart and skin contains 5 exons. What phenomenon likely explains this result?

A) there is a brain- specific allele compared to the other cells in the mouse
B) the skin, liver and heart cells differentiate before the brain and so have different exons
C) the mRNA in the brain is a pseudogene
D) the brain cell chromosome lacks the exon that is present in the other cells of the mouse
E) cells of the brain splice this mRNA differently than other cells in the mouse