Answer :
I think A and D too. The primary purpose was always to protect the Chinese Empire from the Mongolians and other invaders. In 221-206 BC China's first emperor, Qin Shihuang, linked the walls of the three states in the north forming the Great Wall. Though it was eroded and been rebuilt, modified and extended It's over 6,000 km (3,700 miles) long.
And C is wrong because the Qin bulit some and the Ming Dynasty bulit some as well.
And C is wrong because the Qin bulit some and the Ming Dynasty bulit some as well.
I think its D because the great wall is still being build today because of the constant breaks.