Thang-ga(唐卡）is a special kind of painting .It was found only in Tibet,China. It started from the Tang Dynasty(唐朝). Thang-ga paintings show Tibetan people's lives. In recent years, Tang-ga paintings have caught the public's attention .At the same time,the Chinese government has also tried hard to protect the art.

    The color of Tang-ga paintings looks wonderful and special. Because of the special environment of Tibet and the excellent skills of artists, the paints(颜料) used in Thang-ga are different from other paints. They were made from minerals like gold and silver, as well as some plants. The paints were made by hand and the beautiful colors could last(持续)for hundreds of years.

    However, with the development of technology, people now use cheaper and more easily-made chemicals to make Thang-ga paints. The traditional paints used in Thang-ga paintings took a lot of work and they were made in many secret ways. Later people stopped making the paints and forgot about them. So the skills needed to make them were almost lost. The traditional Thang-ga paintings are difficult to find now.

幂函数f（x）=x^α的图象经过点 （4，$\frac{1}{2}$） ，则 $f\left(\frac{1}{4}\right)$ 的值为（  ）

下列语句中，标点符号使用正确的一项是（   ）

一静止的铀核放出一个α粒子衰变成钍核，衰变方程为 ${}_{92}^{238}$ U→ ${}_{90}^{234}$ Th+ ${}_2^4$ He，下列说法正确的是（   ）

 ____期间，帝国主义国家忙于战争，暂时放松了对中国经济的掠夺，中国民族工业出现了“____”。