7. Which of the following best completes the passage below?

The unemployment rate in the LACE w:st="on">United StatesLACE> fell from 7.5 percent in 1981 to 6.9 percent in 1986. It cannot, however, be properly concluded from these statistics that the number of unemployed in 1986 was lower than it had been in 1981 because______

(A) help-wanted advertisements increased between 1981 and 1986

(B) many of the high-paying industrial jobs available in 1981 were replaced by low-wage service jobs in 1986, resulting in displacements of hundreds of thousands of workers

(C) in some midwestern industrial states, the unemployment rate was much higher in 1986 than it had been in 1981

(D) the total available work force, including those with and without employment, increased between 1981 and 1986（D）

(E) the average time that employees stay in any one job dropped during the period 1981 to 1986

选错了，因为不太理解unemployment rate 到底是如何算的（在USA）

谢谢了