Which of the following is an algal parasite?

Read carefully the given statements about algae and choose the correct option.
$I$. The plant body is thalloid.
$II$. Mainly aquatic.
$III$. Reproduction takes place by vegetative,asexual,and sexual methods.
$IV$. Volvox and Ulothrix are the colonial forms of algae.

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

Which type of algae is found at the greatest depth in the ocean?

Match the following columns:

Column-$I$	Column-$II$
$P$. Green Algae	$I$. Ulothrix,Spirogyra
$Q$. Red Algae	$II$. Gracilaria,Gelidium
$R$. Brown Algae	$III$. Ectocarpus,Sargassum

$Spirogyra$ is a