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

Which one of the following groups of animals is correctly matched with its characteristic feature without any exception?

Which of the following has a cartilaginous endoskeleton?

Which of the following possesses a poison sting and belongs to the class $Chondrichthyes$?

Which of the following statements are not true?
$a$. Dermal scales of fishes form the exoskeleton.
$b$. Cycloid and ctenoid scales have distinct lines of growth which aid in determining the age of the fish.
$c$. Fish scales are never shed.
$d$. The skin in elasmobranchs is covered with placoid scales.
$e$. The development of a placoid scale is not like the development of a tooth.

Elasmobranchs are exclusively