题目

The flag, the most common symbol of a nation in the modern world, is also one of the most ancient. With a clear symbolic meaning, the flag in the traditional form is still used today to mark buildings, ships and other vehicles related to a country. The national flag as we know it today is in no way a primitive artifact. It is, rather, the product of thousands of years’ development. Historians believe that it had two major ancestors, of which the earlier served to show wind direction. Early human beings used very fragile houses and boats. Often strong winds would tear roofs from houses or cau当m取何实数时，复数（1）是实数？（2）是虚数？（3）是纯虚数？