125. Read the following passage, then answer the question below.

People often believe that they can do two things at once, such as writing a report while answering messages. Cognitive scientists, however, have found that the brain rarely performs two demanding tasks simultaneously. Instead, it switches rapidly between them, and each switch carries a cost. In a classic 2001 study, Joshua Rubinstein, David Meyer, and Jeffrey Evans asked volunteers to switch between tasks such as solving math problems and classifying shapes. Participants were slower and made more errors whenever they changed tasks, and the cost increased when the tasks were complicated or unfamiliar. Individually these delays amounted to fractions of a second, but across a workday they add up to considerable lost time.

The effects may go beyond speed. In 2009, researchers at Stanford University compared people who frequently used several media at once with those who did so rarely. Surprisingly, the heavy multitaskers performed worse on tests of filtering out irrelevant information and of switching between tasks, the very skills they might have been expected to have sharpened. The researchers could not tell whether heavy media use dulls attention or whether people with weaker attention are drawn to multitasking, but the finding challenged the assumption that practice makes perfect.

There are exceptions. A small proportion of people, about two and a half percent in one driving-simulator study, performed well on both tasks at once; the researchers nicknamed them 'supertaskers.' But most people who believe they belong to this group are mistaken: in the same study, those who most often used phones while driving were generally not the best at it.

The practical lesson is not to avoid all multitasking. Walking while talking, or listening to music while cleaning, involves one task that is largely automatic. The trouble arises when two tasks both require conscious thought. In such cases, completing one task before beginning another is usually faster and more accurate.

What were the researchers unable to determine?