A、Dull. B、Horrible. C、Tranquil. D、Cool. A细节题。短文最后提到，误用蓝色会令人不适，如果没有暖色的搭配，蓝色甚至会显得冰冷而乏味(cold and sterile)，故答案为A)。

Blue is the world’s favourite colour. It is also the colour most often associated with intellect and authority.
    Most uniforms are blue. In Greek and Roman mythology, blue is the colour of sky gods. In the Old Testament, God is represented by deep blue. Blue is the dominant colour in the mosques of the world.[19]Blue symbolises truth, peace and cooperation. And that’s why it is the colour of the flag of the United Nations and of Europe.
    As in the sky and water that surround us, blue is seen as a peaceful and refreshing colour. Blue light has seen to lower blood pressure by calming the nervous system hence relaxing the body and mind.[20]Blue creates large airy spaces. It makes rooms bigger.
    Blue bedrooms are restful. Blue bathrooms are appropriately watery. Blue acquires depth with greens and reds. Light and soft blue makes us feel quiet and protected from the bustle and activity of the day. Dark blue represents the night making us calm. Its apparently calming effect makes it the perfect tone for the quieter zones of your living space.
     [21]The wrong shade of blue can be uncomfortable. It can also be cold and sterile unless balanced with warmer colours.
19. Why does the United Nations use blue as the main colour of its flag?
20. What effect does blue have, according to the passage?
21. Without warmer colours, what impression would blue leave?