问题
Firms that use computers have found that the number ofstaff _____ is needed for quality control can be greatly reduced
A:I studied French for two years
B:I have been studying French
C:I have studied French for two years
D: I had studied French for two years
B:I have been studying French
C:I have studied French for two years
D: I had studied French for two years
正确答案是：
