The argument given relies on which of the following as an assumption? A. No other species of tree started growing in the bed of Lake Tomwa after 1200. B. No tree remains of any kind are present at the bottom of deeper parts of Lake Tomwa. C. There was at least one tree in the lake bed that was alive for the entire period from 1200 to 1320. D. There has not been a more recent drought that caused a drying up of the shallow end of the lake. E. The shallow end of the lake had been dry for less than 30 years by the time Jeffrey pines started growing in the lake bed.