Read the passage about acupuncture.

acupuncture is an alternative medicine that uses wire-thin needles inserted by a trained practitioner into specific points in the body. this ancient form of pain relief and stimulation is said to benefit patients by blocking channels which practitioners claim allow pain to travel within the body. this practice became more widely recognized and used after a new york times reporter wrote in 1971 of a positive experience with it after surgery.

those who doubt the effectiveness of this treatment for pain attribute the pain relief to a placebo effect: if a patient believes something will reduce their pain, they will experience some relief. the national institutes of health says that acupuncture relieves pain and reduces nausea and vomiting, although it is unclear how it works. the few adverse effects reported are due to improperly sterilized needles or improper delivery, making this still a fairly low risk option. for anyone considering acupuncture, they should first research its effectiveness for their specific condition.
summary of statements published by the national institutes of health

after reading the passage about acupuncture, identify which scientific claim explains why some people say acupuncture does not truly relieve pain.
it blocks pain channels.
it is unclear how it works.
it provides only a placebo effect.
it is ancient medicine, not modern medicine.