The correct answer is A. negative.
Cross elasticity of demand is a measure of how responsive the demand for one good is to a change in the price of another good. When two goods are complementary, a decrease in the price of one good will lead to an increase in the demand for the other good, and vice versa. This is because complementary goods are used together, so if the price of one good decreases, people will be more likely to buy it, and they will also be more likely to buy the other good that it is complementary with.
For example, if the price of gasoline decreases, people will be more likely to drive, and they will also be more likely to buy cars. This is because gasoline is a complementary good to cars.
The cross elasticity of demand for complementary goods is always negative. This means that if the price of one good decreases, the demand for the other good will increase, and vice versa.
Option B, zero, is incorrect because the cross elasticity of demand for complementary goods is always negative.
Option C, infinite, is incorrect because the cross elasticity of demand for complementary goods is always negative, but it is not infinite.
Option D, unitary, is incorrect because the cross elasticity of demand for complementary goods is always negative, but it is not unitary.