Vernon is studying the population of deer in a tract of forest. He gathers the following data. Birth rate: ~50 deer/year Death rate: ~70 deer/year Which additional observation would support the conclusion that the deer population is NOT changing size from year to year, meaning that the net population change is 0? A. About 20 deer per year immigrate to the tract of forest. B. About 20 deer per year emigrate from the tract of forest. C. The average life span of a member of the deer population is 20 years. D. If a deer survives its first year of life, its average life span is 20 years.