$A$ group of researchers procured some fish-like animals and upon investigation the following characters were observed: $A$. Endoskeleton was made of cartilage. $B$. Ectoparasitic; as they were found attached on fish skin with their circular sucking mouth. $C$. Paired fins and scales were absent,but $7$ pairs of gill slits were present. Which of the following species of animals did they consider to fit best with these characters?

• A
  $(1)$ $Scoliodon$ sp.
• B
  $(2)$ $Exocoetus$ sp.
• C
  $(3)$ $Petromyzon$ sp.
• D
  $(4)$ $Branchiostoma$ sp.