What is the function of the 19S cap in the proteasome complex?

A) Recognizing the proteins that will be degraded
B) Unfolding the proteins that will be degraded and threading them into the central 20S cylinder for digestion
C) Removing the polyubiquitin chain from the proteins that will be degraded
D) Hydrolyzing ATP to make the digestion process highly efficient
E) All of the above