When a submarine decides to dive, the valves in the outer hull are (a) opened
to let the (b) air out.
The (c) sea water will rush into the ballast tanks.
The weight of the submarine becomes (d) heavier and sink than the buoyant force.
So the submarine sinks into the sea.
When the submarine decides to rise from the water, compressed (e) air is pumped into the ballast tanks to force the (f) sae water out through the valves.
The weight of the submarine becomes (g) lighter and rises than the buoyant force . So, the submarine rises.