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In the dead of winter, we don't typically think about spending time outdoors, but according to many physicians we should. The sun is our greatest source of a vitamin that is essential for good health—Vitamin D.

This important vitamin strengthens bones and teeth and is linked to the prevention of heart disease, cancer and many other life­threatening diseases.The recommended daily requirement is 400­600 International Units (IU). In summer, people with dark brown eyes and darker skin turn sunlight from just 15 minutes outside without sunscreen into adequate amounts of Vitamin D.

Even during winter£¬just a 10­minute walk at midday can keep the body's Vitamin D at healthy levels. During brief winter walks, we receive the greatest benefits when we wear no sunscreen and expose as much skin to the sun as possible.

Vitamin D can be also obtained from foods. Salmon is one of the best sources of the Vitamin. Egg yolks (µ°»Æ)£¬cheese, mushrooms and some animal organs contain smaller quantities of it.

To attract consumers, food companies add Vitamin D to some of their milk products. The practice of putting the Vitamin into milk began in the 1930s. At that time, many children were suffering from rickets, an illness that is caused by a deficiency (ȱ·¦) of Vitamin D£®It made the children's bones soft and weak.Adding Vitamin D to milk helped to solve the problem, and it has been available ever since.

For people who still need more Vitamin D, there is another option£ºTake Vitamin D supplements (²¹³äÎï)£®These products are widely sold in stores.However, anyone considering taking supplements should consult a physician first.

21£®During the winter months, people________.

A£®aren't able to meet their deadlines

B£®often don't dress warmly enough

C£®generally like to remain indoors

D£®leave lots of work for others to do

22£®According to the text, Vitamin D ________.

A£®can be obtained by being exposed to the sun

B£®can be used to cure life­threatening diseases

C£®can be mainly found in some milk products

D£®can make the children's bones soft and weak

23£®Why is salmon mentioned in this article?

A£®It's seldom prepared in the correct way.

B£®It's a useful addition to a person's diet.

C£®It has a huge impact on the environment.

D£®Its advantages have been exaggerated.

24£®The main purpose of this article is________.

A£®to introduce a valuable nutrient

B£®to advertise Vitamin D supplements

C£®to note risks associated with a nutrient

D£®to explain how a nutrient was discovered