In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.

Article: No one can believe that the over 6 300-kilometer long Great Wall might disappear some day.Believe it or not,the Great Wall is being destroyed by people.Less than 20 percent of the Great Wall built in the Ming Dynasty is still perfect,but about 80 percent is in danger.The Great Wall can be called "great" mostly because of its amazing length.But we should realize that the length was made up of one brick at a time.If we do nothing to save the Great Wall,it will become a series of separate wasteland rather than a historic site. The Great Wall is actually a series of walls built and rebuilt by different dynasties over the past 2 000 years.It began in the rule of China's first emperor,Qin Shihuang of the Qin Dynasty (221 BC~206 BC),and lasted into the Ming Dynasty.The parts built before the Ming Dynasty have nearly disappeared.People are familiar with sections such as Badaling in Beijing and Jiayuguan Pass in Gansu because they have been open to tourists for many years.But those sections far away from the public eyes have been almost forgotten. Few local people knew the 3-meter-high walls made of earth and stones beside them are part of the Great Wall.The lack of knowledge is considered as one of the main reasons behind human. The bricks on the Great Wall are carried off by countryside people to build their houses,sheep corral and pigsties .Some were taken away to build roads.Bricks carved with people's names are put away as remembrance.Rubbish is spread over the battlements .The bricks can be sold for 15 yuan per tractor load.Those who destroyed and are destroying the Great Wall know its name,but are not clear about its cultural meaning.It will take a long time to let them know this.The local farmers not only carried off the body of the Wall but dug out the entire base. It is necessary to protect the Great Wall.First of all,the officials should be aware of theimportance of the Great Wall.Young Chinese should know more about the nation's great civilization and learn to love it.
Answer: it had a long history
The main reason for which the Great Wall can be called"great"is _ .