Which of the following explains that the price increase of oil cause the price of natural gas to increase? 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 ?
I think your answer is OK.


A doesn't connect prices in any way. In a two-goods economy and given that wealth doesn't change... you may give it some more thought but you need to add extra assumptions.
B tells us nothing on why such change would take effect. It may just as well be that oil is more valued.
C is the right answer
You need more assumptions for D, e.g. why the oil price increased. At least in the really short term, though, supply isn't a factor.