Natural unemployment equals unemployment plus unemployment. frictional; cyclical frictional; structural cyclical; structural frictional; structural unemployment plus cyclical

I believe the answer is: frictional; structural
Frictional unemployment refers to the type of unemployment that created due to transition that make a person moves from one job to another.
Structural unemployment refers to the type of unemployment that created due to the mismatch of skills between the workers and the job.

The changes to agriculture in the united states since 1900 is that increased mechanization has increased food production, thereby increasing the need for labor. there has been an increased need for higher speeds of food production due the fact that the population in the united states continues to rise.