Sea weeds are an important source of

Which of the following organisms cannot fix nitrogen?
$A.$ Azotobacter
$B.$ Oscillatoria
$C.$ Anabaena
$D.$ Volvox
$E.$ Nostoc
Choose the correct answer from the options given below:

Algae float in the water during day time and sink during night time because

$A$: Algae are autotrophic in terms of nutrition.
$R$: Algae contain chlorophyll and other photosynthetic pigments.

Which one of the following statements concerning the algae is wrong?

$A$ research student collected a certain alga and found that its cells contained both chlorophyll $a$ and chlorophyll $d$,as well as phycoerythrin. The alga belongs to: