A:Hi,Dave.Do you know that the Great Wall is the longest wall of the world?

B:Yes.And I really admire your____They were so clever and brave.

A:What do you think it was used for?

B:It might be used for keeping the wind from____the sand away.

A:Oh,no.There used to be many wars in the north of China.So people built the Great Wall to prevent____ from entering China.

B:Oh,I see.Its____ was to protect people's safety,wasn't it?

A:Yes,you are right.At that time,the leader of China was Qing Shihuang.Thanks to the Great,he got ____of the wars and joined six countries into a big country.

B:He must be the greatest emperor in Chinese history.

A:No,he ____ be the greatest one.In fact,the people lived a hard life at that time.

B:What was the Great Wall made ____?

A:Bricks and stones.Some of the stones are so huge that they ____ as much as two tons.

B:Really?I wonder how they managed to move and____up those heavy stones without modern machines.

A:Yes,there remain many____ about it waiting for us to discover.