Look at the four squares,A ,B ,C , and D, which indicate where the following sentence could be added to the passage．Where would the sentence best fit？
For example, they might estimate the number of black bears in a national park by counting individuals in a few sample plots representative of the whole park．
An important characteristic of any population is its density．A Population density is the number of individuals per unit, such as the number of maple trees per square kilometer in a county．B Ecologists can rarely determine population size by actually counting all individuals within geographical boundaries．C Instead, they often use a variety of sampling techniques to estimate densities and total population sizes．D In some cases, they estimate population size through indirect indicators, such as the number of nests or burrows, or signs such as tracks or droppings．