Why isn’t the natural rate of unemployment equal to zero?

Question

A question on my homework to Macroeconomics. I will give you 'best answer' if you reply.

Answer 1

Because:

1: any economy need some frictional unemployment for labor to find it's best-use employment

2: since any economy is dynamical process so it requires some structural unemployment to meet structural changes in economy (for instance change old profession which is not demanded on market anymore to new one, etc).