Read the following statements and select the correct options that are true for cyanobacteria : 
(A) The colonies are generally surrounded by gelatinous sheath.
(B) They form blooms in polluted water bodies. 
(C) They show nitrogenase activity in heterocyst.
(D) They perform oxygenic photosynthesis in vegetative cell only.

• A

  (A) only
   

• B

  (A), (B) and (C) only
   

• C

  (B) and (C) only
   

• D

  (A), (B), (C) and (D)