These TV sets have gone up in quality, but have not risen in price.but,后主语为什么可以省略

问题描述:

These TV sets have gone up in quality, but have not risen in price.
but,后主语为什么可以省略

前后主语一致

因为前后的主语一致,所以可以省略
如果 不一致,省略会引起歧义