--When _____ the abacus _____? -- In the sixth century by the Chinese people.
They have just cleaned the window, so the room looks _______________.
--How does Jack usually go to school?---He _____________ride a bike, but now he _____________there to lose weight.
-----Could you tell me how long ________________?------ About three days.
I don't know when he . When he , I'll let you know.
—I would like you to talk about the Great Wall.—I’m sorry, but ________ Jack ________ I have been there.