The given characters are seen in which of the following group?
$(i)$ Unicellular,colonial,filamentous,symbiotic or free living.
$(ii)$ The colonies are surrounded by a gelatinous sheath.
$(iii)$ Some can fix atmospheric nitrogen in specialized cells called heterocysts.
$(iv)$ They often form blooms in water bodies.

• A
  Archaebacteria
• B
  Cyanobacteria
• C
  Chrysophytes
• D
  Dinoflagellates