William, this is the solution:
• Step 1: The submarine is sitting still along the surface of the ocean.
• Step 2: As a part of a drill, it dives at a constant speed to a certain number of feet below the surface
• Step 3: Then travels for a time at that depth
• Step 4: When the drill is complete, it ascends at a constant speed to just below the surface
• Step 5: It continues travelling at that depth.
The graph that matches the description that compares time to the submarine's depth, in feet below the surface of the ocean, after it started its dive is:
D. As you can see, it follows step 1 to 5.