The density of Douglas firs in an old-growth forest is estimated by counting the Douglas firs in four sample plots of one hectare each. The number of fir trees in the plots is 10, 12, 7, and 11, respectively. What is the estimated density of firs in the forest? a. b. c. d. 10 trees per hectare 2.5 trees per hectare 20 trees per hectare 5 trees per hectare