The figure shows four animals (a), (b), (c) and (d). Select the correct answer with respect to a common characteristic of two of these animals.
(a) 
(b) 
(c) (d) 
(a) and (d) have cnidoblasts for self-defence
(c) and (d) have a true coelom
(a) and (d) respire mainly through body wall
(b) and (c) show radial symmetry
