The great wall is made of bricks, rocks, and earth. it is one of the biggest construction projects ever finished. the wall meanders through the mountains of northern china. it was originally about 3,000 miles long. its walls are 15 to 30 feet thick and up to 25 feet high. during the ming dynasty (1368–1644), the great wall was lengthened to about 4,000 miles. it was also renovated over a period of 200 years. in that time, cannons were inserted along the wall. watch towers were also added. the great wall is a national treasure to the chinese and one of the most famous tourist destinations in the world. it is estimated that as many as 10 million people visit it each year. unfortunately, the wall has suffered greatly in recent years from damage and neglect. in 1987, the great wall was named a world cultural heritage site by the united nations educational, scientific and cultural organization (unesco). 6 which of the following sentences should be added to the first paragraph to support the idea that the great wall is one of the largest building projects ever built? a. the great wall is so immense that it can be seen by astronauts orbiting around the earth. b. in recent history, accounts indicate that 150 million tourists have visited the great wall of china. c. for centuries in china, soldiers placed signal fires along the great wall to warn of approaching invaders. d. the wall is 9 to 12 feet wide at the top, wide enough for troops of soldiers or wagons.