As far as I know, the robot that can sweep the floor ________ in the 1980s.( )
A. |
is invented |
B. |
invented |
C. |
was invented |
D. |
has invented |
As far as I know, the robot that can sweep the floor ________ in the 1980s.( )
A. |
is invented |
B. |
invented |
C. |
was invented |
D. |
has invented |