Pop quiz: Fill in the blanks.

Review

Here is ____ astonishing and significant fact: Mental work alone can’t make us tired. It sounds absurd. But a few years ago, scientists tried to find out how long the human brain could labor without reaching a stage of fatigue (疲劳).To the ___________ (amaze) 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, 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. So far as the brain ____________ (concern), it can work as well and swiftly at the end of eight or even twelve hours of effort ___ at the beginning. The brain is totally tireless. So what makes us tired?

an

amazement

as

is concerned