Lateral line sense organs occur in

Which of the following statements are not true about shark?
$a$. The skin is tough and covered with placoid scales
$b$. They possess a swim bladder which regulates buoyancy
$c$. The tail of shark is heterocercal
$d$. Ampulla of Lorenzini present in the snout of shark is a thermoreceptor.
$e$. It has five pairs of gill slits covered by operculum

Electric organ is present in

$S$ - Statement: Shark is an example of cartilaginous fish.
$R$ - Reason: The endoskeleton is made of cartilage.

Which of the following are the first Gnathostomes?

What is the location and function of claspers?