Oil and natural gas are substitute goods. A tax on oil will have what effect on the market for natural gas? The supply of natural gas will increase, causing its price to fall. The demand for natural gas will increase, causing its price to rise. The supply of natural gas will decrease, causing its price to rise. O Both the supply and demand of natural gas will decrease, causing its price to fall. Both the supply and demand of natural gas will increase, causing its price to rise. The demand for natural gas will decrease, causing its price to fall