They explained very clearly. But I just wanna say that depends on the context of "龙江颂“ and the social backgrounds of that special time, "公” is better to be explained as country and the public collective. At that time, after the WWII and civil war which China suffered from, China's economy, markets and the basic industries were all destroyed and ruined in these two Wars. So planned economic system was the inevitable choice. That means almost everything belonged to the collective, and the public collective decided to distribute most things to every family. So "私" may not be that selfish like its modern meaning. It should be explained as keeping something for themselves as their personal properties. ... This work "龙江颂" mirrored the features of the age.