A sample of insects and other invertebrates is collected from an urban stream. A total of 244 individuals is collected from 2 m2 of stream bottom habitat. In this sample there are 22 recognizably different types of invertebrate. The average total length of the individuals is 6.7 mm. What is the best estimate of species richness based on these data?
a. 122
b. 244
c. 22
d. 6.7