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.
Scientists have developed a new bionic computer chip that can be mated with human cells to combat disease. The tiny device, smaller and thinner than a strand of hair, combines a healthy human cell with an electronic circuitry chip. Doctors can control the activity of the cell by controlling the chip with a computer.
It has long been established that cell members become permeable when exposed to electrical impulses. Researchers have conducted genetic research for years with a trial-and-error process of bombarding cells with electricity in an attempt to introduce foreign substances such as new drug treatments or genetic material. They were unable to apply a particular level of voltage for a particular purpose. With the new invention, the computer sends electrical impulses to the chip, which triggers the physicians to open a cell‘s pores with control.
Researchers hope that eventually they will be able to develop more advanced chips whereby they can choose a particular voltage to activate particular tissues, whether they be muscle, bone, brain, or others. They believe that they will be able to implant multiple chips into a person to deal with one problem or more than one problem.
The word they in the first sentences of the third paragraph refers to________.