The cross-price elasticity of electricity with respect to the price of natural gas has been estimated as being equal to 0.2. This implies that:
选项：

A:natural gas and electricity are both normal goods.
B:electricity and natural gas are complements.
C:electricity and natural gas are substitutes.
D:one of the two goods is inferior while the other is normal, but we need additional information to determine which of them is inferior.