Answer:
Negative sign
Explanation:
As the problem states, the cross-price elasticity measures how the quantity demanded of good A changes when the price of good B changes by 1%.
It is common that people use shampoo and conditioner, but not only conditioner(maybe only shampoo, but for this example let's suppose that most of the time, people do not use only one) . We can affirm that they are complementary goods. The cross-price elasticity of shampoo and conditioner should be negative because if the price of one increases (for example conditioner), the quantity demanded for the other, (shampoo), will decrease.
