The correct answer is: A. PQRS
The sentence is about how it is difficult to restrain our desires once we set about satisfying them. The first part of the sentence, “it becomes difficult,” sets the stage for the rest of the sentence. The second part of the sentence, “satisfying our desire,” is the main clause of the sentence. The third part of the sentence, “but once we set about,” is a subordinate clause that provides additional information about the main clause. The fourth part of the sentence, “if not impossible to restrain them,” is the conclusion of the sentence.
The other options are incorrect because they do not make sense in the context of the sentence. Option B, QRPS, is incorrect because the subordinate clause “but once we set about” comes before the main clause “satisfying our desire.” Option C, RQPS, is incorrect because the conclusion of the sentence, “if not impossible to restrain them,” comes before the main clause “satisfying our desire.” Option D, RQSP, is incorrect because the main clause “satisfying our desire” comes after the subordinate clause “but once we set about.”