Which of the following has a cartilaginous endoskeleton?

In rays (cartilaginous fish),the mouth is located on which side?

The heart of a fish is different from other vertebrate groups because:

$A$ marine cartilaginous fish that can produce electric current is

Assertion $A$: Sharks are viviparous animals.
Reason $R$: Sharks possess mammary glands.
Which option is correct for Assertion $A$ and Reason $R$?

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