Rearrange the parts of the sentence in correct order.
Accepting that one form
P-of price discrimination is okay opens
Q-the door to all other forms
R-of discrimination as well

Let's connect the fragments logically:
The introductory phrase ends with "one form". This naturally requires a prepositional phrase starting with "of" to identify the form. This points directly to section P: "of price discrimination is okay opens".

The verb "opens" in P expects an object. This flows perfectly into section Q: "the door to all other forms".

Section Q ends with "all other forms", which is then specified by section R: "of discrimination as well".

Combining these gives: P → Q → R.