This is an application of the cross-price elasticity equation: \% change in QDx/\% change in Py. -0.10/0.2 = -0.5. A negative number means the goods are complements. 
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 

(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. 