According to the fifth paragraph, studies have shown that the insertion of acupuncture needles into acupoints causes nerve stimulation that results in____.

A、 signals being sent to the brain and pituitary gland, which leads to the release of chemicals
B、 signals being sent to the spinal cord, which immediately blocks the release of chemicals
C、 chemicals being released that amplify signals to the spinal cord
D、 chemicals being released that numb the spinal cord and prevent signals being sent to the brain and pituitary gland