Which of the following are the first Gnathostomes?

Which of the following animals possesses an endoskeleton made entirely of cartilage?

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 organisms possesses a closed circulatory system?

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

The Ampulla of Lorenzini in fishes is a: