Question 16-20 are based on the following talk.
Today, I’m going to talk about the health problems related to sun-tan. Sun-tan is preferred by lighter-skinned people, especially during the summer months. They like to give their skin a nice natural bronze color with the help of sun-tanning. Getting a natural tan is the sign of being healthy and being attractive. No wonder many people rush off to the beach at the onset of summer with their tanning lotions and laze away on a towel for hours on end. The result is healthy tanned skin, which people like to flaunt all day long. People often believe that a perfectly tanned skin improves their personality and the texture of their skin. However, it is not advisable to be in direct sunlight for a long time. Excess exposure of the skin to sunlight can actually damage the skin, and is the leading cause of skin cancer all around the world. Moreover, in countries like Australia, the atmosphere does not filter the sun rays completely, as the Ozone layer in the atmosphere is depleting quickly. The Ozone layer is responsible for filtering the harmful ultra-violet or UV rays of the sun that can damage the skin beyond repair.

So what does a tan mean? A tan refers to exposure of the skin to the rays of the sun for a considerable amount of time. The UVB rays of the sun encourage the cells of the innermost layer of the skin, to produce more melanin pigments, which on their way to the outmost layer are tanned by the UVA rays of the sun. As a result, Vitamin D3 is produced naturally under the sun. The vitamin helps protect the bone and also protects us from diseases like osteoporosis. The direct rays of the sun also make the immune system stronger against the invasion of germs, and increase the body’s overall physical powers. It has a positive effect on the functioning of the heart, and improves blood circulation. Common skin diseases like acne and neurodermatitis can be treated with ultra-violet light. These are just some of the many positive effects that the sun offers in the form of its ultra-violet rays. You should take the advice of experts if you would like to go for a natural sun-tan. Different skins react differently to sunlight, and expert opinion should be sought, especially in the beginning. Do not overdo a sun-tan, as excess exposure to the sun’s rays can prove to be harmful to your skin. Wear a sun-tan lotion when you go out in the open during summer. Do not sunbath for more than once a day. Going to the beach or lying out in the sun once or twice a week is enough to maintain a good tan on the skin. Tanning in summer proves to be very beneficial, as it prevents skin fatigue, prepares your skin for the long summer months ahead, builds a natural protection, and produces a nice natural tan.

Questions:
16. What is the main topic of the speech?
17. Which of the following is responsible for filtering the ultra-violet rays of the sun?
18. What is the specific function of Vitamin D3?
19. The ultra-violet rays of the sun have several positive effects on human health. Which of the following is not one of these effects?
20. What can we conclude from the speech?

评析：这是一篇关于健康的文章。文章主要讲述了晒日光浴的好处，以及在晒日光浴时要注意到的健康问题。话题不算难，有部分考生可能会受词汇影响而耽误听力理解。以下这些词是“日光浴”这个话题延伸出来的高频词汇：
deplete: 消耗，耗尽，减少
exposure：暴露
Ozone layer: 臭氧层
filter: 过滤
ultra-violet：紫外线
osteoporosis：骨质酥松症