Answer:
D. The flow rate of the large tank is -20 gal/min, and the flow rate of the small tank is 20 gal/min.
Explanation:
Flow rate =

flow rate of water in the big tank =

= 60 gallons/minute
flow rate in the smaller tank =

= 40 gallons/minute
The difference in flow rate between the tanks = 60 - 40
= 20 gallons/minute
Since the two tanks have the same amount of water after the given time, then the volume of water in the large tank decrease; while the volume of water in the small tank increased.
Therefore, the flow rate of the large tank is -20 gal/min, and the flow rate of the small tank is 20 gal/min. This shows that option D is correct.