Which is the most venomous fish?

Cartilaginous fishes belong to the class

Which type of scales are found on the skin of cartilaginous fishes?

Which of the following characteristic features always holds true for the corresponding group of animals?

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

Which of the following is $NOT$ a characteristic feature of the group $Pisces$ (fishes)?