--When _____ the abacus _____?   -- In the sixth century by the Chinese people.
#44867
A
is, invented
B
did, invent
C
was, invented
D
does, invent
推荐题目
---It’s difficult for the villagers to cross the river. ---I think a bridge______ over the river.
#44765
A
should be built
B
is being built
C
has been built
D
was built
Zhengzhou will be more and more convenient when Zhengzhou Subway_____ in two years.
#44912
A
is completed
B
will be completed
C
will complete
D
has completed
 I think students _____ to do homework with friends.
#44494
A
should allow
B
should be allowed
C
were allowed
D
must be allowing