The correct answer is: B. Supply
In the very short run, supply is the main factor that determines price. This is because the quantity supplied can change quickly in response to changes in price, while the quantity demanded takes longer to adjust. For example, if the price of a good increases, producers will be able to increase the quantity supplied relatively quickly by hiring more workers or using more capital. However, consumers may take longer to adjust their demand for the good, as they may need time to find substitutes or to adjust their budgets. As a result, the price of the good will tend to rise in the very short run, until the quantity demanded catches up with the quantity supplied.
Option A is incorrect because demand is not the main factor that determines price in the very short run. As explained above, supply is the main factor that determines price in the very short run.
Option C is incorrect because both supply and demand are not the main factors that determine price in the very short run. As explained above, supply is the main factor that determines price in the very short run.
Option D is incorrect because supply plays a dominant role in price determination in the very short run.