Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 31 to 35

Deep brain stimulation (DBS), an experimental technology that involves implanting a pacemaker-like device in a patient’s brain to send electrical impulses, is a hotly debated subject in the field of medicine. It is an inherently risky procedure and the exact effects on the human brain aren’t yet fully understood.

But some practitioners believe it could be a way to alleviate the symptoms of depression or even help treat Alzheimer’s — and now they suspect it could help with drug addiction as well. In a world’s first, according to the Associated Press, a patient in Shanghai’s Ruijin Hospital had a DBS device implanted in his brain to treat his addiction to methamphetamine. And the device has had an astonishingly positive effect, the patient says. “This machine is pretty magical. He adjusts it to make you happy and you’re happy, to make you nervous and you’re nervous,” he told the Associated Press. “It controls your happiness, anger, grief and joy.”

Other studies in China have yielded mixed results trying to treat opioid addictions using DBS, according to the AP. In the United States, at least two studies that tried to treat alcoholism with DBS were dropped for not being able to justify the risks. The idea of using DBS to treat drug addiction has raised concerns in medical communities across the globe about brain hemorrhage, seizures, or personality changes

The word “It” in paragraph 3 refers to__________________