Answer: Atmosphere and geosphere.
Geo-sphere is the solid part of the earth. Hydrosphere is the water part. The living things on the earth make up the biosphere. The gases, water vapors etc make up the atmosphere. When a volcanic eruption adds carbon dioxide to the air, the carbon-dioxide is being added from geosphere to the atmosphere. Hence, there is interaction between atmosphere and geosphere.
The water flows the fastest at the center.
This is due to the friction between the water and the sink/drain. The outer parts of the water that are in contact with the kitchen sink/drain experience a greater frictional force and slow down. Their slowing down also affects the layers of water molecules adjacent to them, thereby reducing their speed as well. The layer of water molecules least affected by the friction with the sink/drain is at the center, and so they are the ones that move the fastest.
