The sun, the moon and the stars(星星)are all in the sky. We see the moon and the stars at night. The sun is a ball. It's much bigger than the moon. It gives us heat(热量). The sun has a family. It's the biggest member of the family. The moon is a ball, too, like the sun and the earth. Sometimes it looks as round as a plate(盘子). Sometimes it looks half a plate. It looks bigger because it's much nearer to the earth. It's much smaller than the sun. It's smaller than the stars. The light on the moon comes from the sun. It has no light of its own.
   The stars look smaller than the sun. They look small because they are very far away from us. All day the stars are in the sky. But the light from the sun is too bright. We can't see the stars in the day.

在阴雨天气下，当电场强度超过3×10⁶V/m时，空气将被击穿而发生放电。现有一片带电的乌云，距地面300m高，发生闪电时乌云与地面的电势差至少有（   ）

假如你生活在宋代，不可能做到的是（    ）