Question 21 is based on the passage you have just heard.

听力原文

        (19) [Sunshine and high temperatures can mean picnics and parks and trips to beaches, but they can make your body react in some strange ways.] First, sunshine can actually affect your breath. You may think that summer is the perfect time to go on a date or chat with an attractive boy or girl, (20) [but excessive heat can cause you to lose water, which has been proven to have a pretty unattractive side effect—bad breath.] When you are lacking water, your mouth becomes dry. Bacteria accumulates because there isn’t enough water in your mouth to wash it away. Sunshine can stop you from sweating too. The body’s normal temperature is 36 to 37℃, but if it heats up to 40 to 41 degrees, you may find yourself getting heat stroke. At that point, your heat regulatory system can essentially quit, which stops you from sweating as your body tries to keep water around your vital organs. (21) [However, the sun can have a positive influence on your body by raising your energy levels. You might feel like the sun is giving you extra energy for your day, which is actually happening.] And vitamin D from sunshine can help your body to absorb certain minerals, which are linked to building stronger bones. Therefore, I suggest you spend more time in the park with your friends, but make sure you apply lots of skin protection, and fill up those water bottles too.

21. What is said to be a positive effect of sunshine?（文章说阳光有什么积极作用？）

解析：B。录音后半部分提到，太阳可以通过提高人体的能量水平（raising your energy levels）来对身体产生积极影响。后面又提到，太阳会为你提供一整天的额外能量（giving you extra energy for your day）。B项与此内容相符，故为正确答案。

错项排除：A项利用录音中的bacteria设置干扰，但其中的protects people无依据，故排除。C项和D项内容在录音中未提及，故均可排除。