9708/13 Economics May Jun 2022 Online Test | Cambridge AS and A Level MCQ

In the diagram, D is the demand curve of an agricultural commodity and S1 is the initial supply curve.
The government introduces a buffer scheme to maintain farm incomes at their initial level.
What quantity would the government need to buy from the farmers to add to the buffer stock if a bumper harvest causes the supply curve to move to ${S_2}$?