A researcher attempts to use a segment of ribosomal RNA as a molecular clock to calculate the date of the last common ancestor of two species. He finds that the two species diverged from a common ancestor 150 million years ago. These particular species have abundant fossil records that strongly suggest that these species diverged 100 million years ago. Which of the following answer choices best describes a reason for the discrepancy in the date of the last common ancestor of these species?

A. Ribosomal RNA is not a reliable molecular clock and should not be used for dating.
B. The neutral mutation rate used in calculating the last common ancestor date may not have been accurate.
C. It is highly likely that the two organisms did not share a common ancestor, so the results are invalid.
D. A large portion of the ribosomal RNA is not vital to survival, so it is not likely to be well preserved.