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.

之所以说鸦片战争是中国近代史的开端，主要依据是

A．中国革命任务的变化

B．中国社会性质的变化

C．中国社会主要矛盾变化

D．中国革命性质的变化