No one knows how the huge rocks   and ___ without our modern machines eight hundred years ago.

— Food problem is becoming a big one in China! —I think the food ________ before going to shops or supermarkets. 

– It’s difficult for the village children to cross the river to get to school.– We think a bridge ________ over the river.

Mr. Li ____ to give a talk in our school yesterday.

--When _____ the abacus _____?   -- In the sixth century by the Chinese people.