$5.$ Based on the class relationships above, indicate the valid or invalid declarations.

a$.$ Animal a $=$ new Animal$()$;

b$.$ Animal b $=$ new Cow$()$;

c$.$ Animal c $=$ new Chicken$()$;

d$.$ Cow d $=$ new Animal$()$;

e$.$ Cow e $=$ new Chicken$()$;

f$.$ Chicken f $=$ new Animal$()$;

g$.$ Chicken g $=$ new Cow$()$;