GRE Reading Comprehension: ETS-GRE阅读ETS - 7ST81L48ZHAM49VL7

The author uses the example of Anabaena to illustrate the A. uniqueness of bacteria among unicellular organisms B. inadequacy of an existing view of bacteria C. ability of unicellular organisms to engage in photosynthesis D. variability of a freshwater bacteria E. difficulty of investigating even the simplest unicellular organisms