Sandra purchases 5 pounds of coffee and 10 gallons of milk per month when the price of coffee is $10 per pound. She purchases 6 pounds of coffee and 12 gallons of milk per month when the price of coffee is $8 per pound. Sandra's cross-price elasticity of demand for coffee and milk is

a) 0.82, and they are substitutes.
b) 1.22, and they are substitutes.
c) -0.82, and they are complements.
d) 1.22, and they are complements