How is D wrong?

Hi Tyler, Lets imagine that patents can cover products that are similar to another invention. Say, I invent a new invention, an internal combustion engine. This engine is very similar to an existing product, the steam engine, They are both engines, they both use fuel to turn a piston, and they both move things from point A to point B. Since these are, by all accounts, similar products, is the internal combustion engine not worthy of a patent? We have no idea what it means for an invention to be similar to another product. Two things can be similar, but different in critical ways.