The first evidence for introns in eukaryotic genes was based on electron microscopy of DNA-RNA hybrids. In the diagram to the right, the dashed line labeled "A" and the double strand loop labeled "B" represent, respectively

A.	unspliced mRNA and exon sequences in DNA
B.	introns and exons
C.	5'-exon and 3'-exon
D.	spliced mRNA and introns sequences in DNA EXPLANATION
E.	mRNA and plasmid DNA