GMAT Sentence Correction (SC) | Re: Trouble understanding SC

The tidal forces to which an object falling into a black hole is/are subjected is/are sufficient to tear the object apart.

So I started with getting rid of the prepositional phrases.

The tidal forces to which an object falling into a black hole is/are subjected is/are sufficient to tear the object apart.

Now I'm left with, The tidal forces is/are subjected is/are sufficient.