State whether or not each of the following equilibrium reactions could have the equilibrium expression.
a) C (g) <-----> 2A (aq) + B (g)
b) 2A (g) + B (g) <------> C (g)
c) C (g) + D(s) <-----> 2A (aq) + B (g)
d) C (s) + D (g) <-----> 2A(aq) + B (aq)