A 20\% increase in the price of milk leads to a 10\% decrease in the quantity of cereal purchased. The cross-price elasticity of demand between milk and cereal is 
选项为:A:-0.5 and the two goods are substitutes.  B:-0.5 and the two goods are complements.  C:0.5 and the two goods are complements.  D:-2 and the two goods are substitutes.  E:2 and the two goods are complements.  请回答其中一个选项.