①电离辐射       ②紫外线       ③煤焦油       ④肿瘤病毒        ⑤苯．

C

    As is known to all, there are 55 minority groups (少数民族) in China, each with its traditional customs and culture. Now let's take a look at the Tibetan Minority.

    The Tibetan Minority groups live in Qinghai, Gansu, Sichuan and Yunnan Provinces. And the highest ridge in the world--beautiful and mysterious Tibet is their major living place. As there are rich grass in Tibet, most people live on raising sheep, goats, cattle and growing a special kind of plant called Qingke. Because of the weather, people often wear warm and comfortable boots in winter, and they always wear robes (长袍) which are made of the fur (毛皮) of sheep. Often they take off one sleeve of the clothes in order to work easily and use it as warm quilts at night.

    Both men and women are good at singing and dancing. When meet an important guest, they will present him a Hada, a piece of long white silk cloth to show their respect(尊重). And they will also treat the guest to a special kind of drink called Suyou Tea.

    The Tibetan Minority has deep emotions. The people there are warm-hearted and friendly. They live freely and happily on the spacious grassland. Their history and culture are very important in China, so we should protect them and make them happy and let them receive great respect.

己知 $\Delta ABC$ 中， $\angle B$ 是 $\angle A$ 的2倍， $\angle C$ 比 $\angle A$ 大20°，则 $\angle A$ 等于____.