Which of the following statements about unemployment is least accurate（）

A. Frictional unemployment is unemployment due to changes in the economy that prevent qualified workers from being immediately matched with existing job openings.

B. The economy is said to be operating at full employment only if there is no cyclical or frictional unemployment in the economy.

C. The economy is said to be operating at full employment when cyclical unemployment is zero.

硫印检验可以分析钢中硫化物分布情况。