To find out how much baking soda Lisa should add to 9 lbs. of flour, we first need to determine the ratio of baking soda to flour based on the initial proportions.
Lisa adds 2 tsp of baking soda to 6 lbs of flour.
We can find the amount of baking soda per pound of flour: \[ \text{Baking soda per pound} = \frac{2 \text{ tsp}}{6 \text{ lbs}} = \frac{1}{3} \text{ tsp per lb} \]
Now, to calculate how much baking soda is needed for 9 lbs of flour: \[ \text{Baking soda for 9 lbs} = 9 \text{ lbs} \times \frac{1}{3} \text{ tsp per lb} = 3 \text{ tsp} \]
So, Lisa should add 3 tsp of baking soda to 9 lbs of flour. The correct answer is:
3 tsp