It is estimated that the price of a personal computer（ ）by an average of 25% a year since 1990.
A.正确答案Dfell
B.had fallen
C.is falling
D.has fallen
It is estimated that the price of a personal computer（ ）by an average of 25% a year since 1990.
It is estimated that the price of a personal computer（ ）by an average of 25% a year since 1990.
A.正确答案Dfell
B.had fallen
C.is falling
D.has fallen
