Explanation: While all of the answer choices are indeed policies Tang emperors used to maintain the frontier zones, the passage needs to support the correct answer. You can eliminate (A) because the passage makes no mention of diplomacy with the frontier tribes. Eliminate (C) because the passage does not state that the soldiers were settled permanently on the frontier, just garrisoned there for a long time. Eliminate (D) because the passage does not indicate that the Tang paid tribute to frontier peoples. Choice (B), the Tang’s ability to project military power on the frontier in order to impose their will, is correct.