Read the following statements and choose the correct option                                             

a) All cyanobacteria are nitrogen fixers

 b) The main sources of biofertilisers are bacteria, fungi and cyanobacteria

c) Azospirillum and Azotobacter are symbiotic nitrogen fixers

d) Microbes like bacteria and many fungi can be grown on nutritive media to form colonies but they cannot be seen with the naked eyes