Given the information in the passage, to which one of the following would lichenometry likely be most applicable?

I felt good eliminating A-C, almost clicked D, then went back to the passage and saw that the referenced a lichen growing at 9.5mm/century and I thought...if lichen is growing that slowly how can we identify a rate in km/century from that? Was it that I should have assumed a much faster growth rate from another lichen + coupled with the fact that E had more assumptions/lack of support.