Final answer:
The best word to complete the sentence is positive. This indicates that ginger tea and lemons have a positive cross elasticity, meaning they are substitute goods.
Step-by-step explanation:
The best word to complete the sentence is positive. From the given information, we can deduce that there was a pronounced increase in the demand for ginger tea and lemons during the height of the second wave of Covid-19. This indicates that the two items have a positive cross elasticity, meaning that they are substitute goods. When the price of one good (lemons) increases, the quantity demanded for the other good (ginger tea) also increases.