Here is a surprising and important fact: Mental work(脑力劳动) alone can’t make us tired. It sounds absurd(荒谬的). But a year ago, scientists tried to find out how long the human brain could work without reaching a stage of fatigue (疲劳). To the amazement of these scientists, they discovered that blood passing through the brain, when it is active, shows no fatigue at all! If we took a drop of blood from a day laborer(工人), we would find it full of fatigue toxins(毒素) and fatigue products. But if we took blood from the brain of an Albert Einstein, it would show no fatigue toxins at the end of the day.
In fact, brain can work as well at the end of eight or even twelve hours of effort as at the beginning. The brain is totally tireless. So what makes us tired?
Some scientists say that most of our fatigue comes from our mental and emotional(情绪的) attitudes(态度). One of England’s most outstanding scientists, J.A. Hartfield, says, “The greater part of the fatigue is of mental origin(源于精神的). In fact, fatigue of physical origin(源于身体的) is rare.” Dr. Brill, a famous American scientist, goes even further. He says, “One hundred percent of the fatigue of sitting workers in good health is because of emotional problems.”
What kinds of emotions make sitting workers tired? A feeling of being bored, angry, nervous or worried, a feeling of not being appreciated(被欣赏)--- those are the emotions that tire sitting workers. Hard work by itself seldom causes fatigue. We get tired because our emotions produce nervousness in the body.
What surprised the scientists a few years ago?

A．Fatigue toxins could hardly be found in a laborer’s blood.

B．Albert Einstein didn’t feel tired after a day’s work.

C．The brain could work for many hours without fatigue.

D．A mental worker’s blood was filled with fatigue toxins.

According to the writer, which of the following can make sitting workers tired?

A．Challenging mental work.

B．Unpleasant emotions.

C．Endless tasks.

D．Physical labor(体力劳动).

What’s the writer’s attitude towards the scientists’ ideas?

A．He agrees with them.

B．He doubts them.

C．He argues against them.

D．He is not willing to accept them.

We can infer from the passage that in order to stay energetic, sitting workers need to ________.

A．have some good food

B．enjoy their work

C．exercise regularly(规律地）

D．discover fatigue toxins