Which of the following statement$(s)$ is/are correct regarding phylum $Coelenterata$ (Cnidaria)?
$(A)$ They are aquatic,mostly marine,sessile or free-swimming,radially symmetrical animals.
$(B)$ They have a central gastro-vascular cavity with a single opening on hypostome.
$(C)$ Digestion is extracellular and intracellular.
$(D)$ Their members are $Sycon$,$Spongilla$ and $Euspongia$.

• A
  $A$ and $B$
• B
  $A$ and $D$
• C
  $A, B$ and $C$
• D
  $A, B, C$ and $D$