The diagram shows the effect of a point mutation on a section of a gene. The nitrogenous base adenine ("A") is inserted into the sequence. (Note: The diagram shows the base sequence of DNA in the top row, and the transcribed mRNA in the bottom row.) The bases of a DNA sequence and corresponding RNA sequence are shown before and after an insertion mutation. The insertion adds A to the DNA and U to the RNA, and it shifts the grouping of three adjacent nucleotides in RNA. What best describes this mutation and its effect on the protein that the gene produces? A. It is a missense mutation that changes exactly one amino acid. B. It is a nonsense mutation that has no effect on the protein. C. It is a frameshift mutation that changes many amino acids. D. It is a frameshift mutation that changes exactly one amino acid.