Solution:"Crowded City side Walk" is non-excludable but rivalry in nature. Excludability: For a typical city sidewalk, its generally very difficult, if not impossible, to prevent people from using it.
There are no tolls, gates or specific acces controls. So it is non-excludable.
Rivalry: If the sidewalk is crowded, then one person's use does diminish another person's ability to use it. When it's crowded, people bump into each other, have to slow down, or can't move freely.
Therefore, a crowded sidewalk is rivalrous. It if were an empty sidewalk, it would be non-rivalrous. But the key world here is "crowded".