A) Oil is inferior good while natural gas is normal good.
B) Both of them are normal good.
C)They are substitute goods and the higher price for oil increased the demand for natural gas.
D)They are substitute goods and the higher price for oil decreased the supply for natural gas.
In my personal opinion, the answer should be C. We can eliminate A and B first as there's no indication of income change. For D think since substitute good refers to the customer side so I think we should also remove D. Is there a better explanation ?