Tulou, the special residential architecture of Fujian Province was included on the UNESCO's World Heritage List during the 32ndsession of the World Heritage Committee in Quebec, Canada. In the fourth century, Han Chinese living in the Central Plains area began to migrate south, gradually gathering in Fujian and forming the Hakka communities. As a defence against enemies, the Hakkas chose to live in compact  communities, and the tulou was their preferred houses. Tens of thousands of such earthen structures were constructed in Fujian Province. Most tulous are to be found in the valleys, surrounded by high mountains, and some are in the depths of the great mountains. Most are three to four stories high, and look like circular blockhouses . Rooms on the first floor are used as kitchens, rooms on the second floor are used as barns , and rooms on the third and fourth floors are for bedrooms and living rooms. For defensive purposes, the rooms on the first floor have no windows. Raw materials for the tulou were obtained locally. Their main building material was a mixture of clay, sand, lime and water, and egg whites, brown sugar and rice water were added as adhesive agents . It was then mixed to form the walls. Once they dried, the walls were so hard that driving a nail into them would have been difficult. Fir branches, which are extremely strong and do not rot, were used to strengthen them, and many centuries later they have remained their original look. Tulous are located in a region where earthquakes happen frequently, and their circular construction helps them resist the regular shocks. The proven design even inspired one famous Peruvian architect, who paid several visits to Yongding, to build a tulou back home. Not long after, an earthquake struck only 10 kilometers away, and while all the houses around the earthen building fell down, his tulou remained. Which of the following is TRUE according to the passage?
Hakkas built tulous when they lived in the Central Plains area Raw materials for the tulou were obtained from far away It is difficult to dig a hole into the walls of the tulou Fir branches may help them resist the regular shocks