Do you celebrate the Mid-Autumn Festival? Do you remember the story ? How did this festival1?

    Long ago, our earth had ten suns. It was too2 for plants to grow, so everyone was poor and hungry.

    Hou Yi climbed to the3 mountain and shot down nine 4 He was a hero! As a reward, the God gave him a special medicine, and it can make him5forever. To keep the6safe, Hou Yi7it to his beautiful wife, Chang'e.

    One day Hou Yi was away. One of his followers, Peng Meng, tried to make Chang'e give him the medicine. Instead, Chang'e had to take it herself, and suddenly she8 up to the moon.

    When Hou Yi returned, he found Chang'e had gone and he missed her. Then he saw 9.It was very bright. A shadow on it looked just like 10.

    He set a table in the garden, with fruits and food for Chang'e.

    Now people still do the same at Mid-Autumn Festival.

多数同学家里都有调光台灯、调速电风扇，过去是用变压器来实现上述调节的，缺点是成本高、体积大、效率低，且不能任意调节灯的亮度或风扇的转速．现在的调光台灯、调速电风扇是用可控硅电子元件来实现调节的，如图所示为一个经过双向可控硅电子元件调节后加在电灯上的电压，即在正弦式电流的每一个 $\frac{1}{2}$ 周期中，前面的 $\frac{1}{4}$ 被截去，从而改变了电灯上的电压．那么现在电灯上的电压为（   ）