48. Read the following sentences from the reading passage. What does “they” refer to? Scientists at Pennsylvania State University in the U.S. conducted a study in which research participants were told that a new technology, which doesn’t actually exist, could measure their sleep quality from the night before. They were connected to a machine monitoring brainwave frequency and shown piles of spreadsheets and formulas. Then some were informed that their deep sleep or rapid eye movement (REM) sleep had been above average the night before, a sign they were mentally alert. (A) scientists (B) research participants (C) formulas (D) rapid eye movements