Step 1: Understanding the Concept:
The sentence has a contrast ("but") and describes a consequence.
Step 2: Detailed Explanation:
Correct order: R-Q-P-S: "but once we set about satisfying our desire, it becomes difficult if not impossible to restrain them." R introduces the action, Q tells what we set about doing, P gives the result, S adds emphasis.
Step 3: Final Answer:
Thus, RQPS is the correct sequence.