Identifying fish and shellfish

This task is about using a classification key to identify animals.
a)
 
 
Use the chart above to identify this fish.
 
 
The fish shown is a ...
(A)   trout.
(B)   kokopu.
(C)   perch.
(D)   carp.
 
 
 
 
 
b)
[NOT DRAWN TO SCALE]
 
 
These molluscs belong to three different groups: A, B, and C.
 
  i)
What feature do all these animals have in common? _______________________
 
  ii)
Complete the spaces i) to vii) so that someone would be able to identify these five animals.
[The question, New Zealand Freshwater Fish, might help you answer this.]