What does each underlined part refer to?

Change is natural, but that doesn't mean we have to start falling apart when we reach a particular age. The primary reason people degrade with age is that they believe (a)they will. Our physical form is a manifestation of our nonphysical self, and it (b)does what we expect it to (b)do. If an authority figure like a scientist or doctor tells people they will degrade because they have reached a certain age, then they are likely to believe it - and they will (d)do just that.

Is (a) interpreted as "they will degrade with age"?

Are (b) and (c) pro-verbs or regular verbs?

Does (d) mean "believe it" or "degrade because they have reached a certain age"?

Thank you so much.