通读全文，理解大意，从各题所给的选项中选择可填入短文的最佳选项。
Nowadays, millions of Chinese leave the countryside to search for work in the cities. They usually go back to1once or twice a year. Zhong Wei is one of them.
Zhong Wei has lived in Wenzhou for the last 13 years. With a hard job in a crayon factory, he is too busy to find2to visit his hometown.
Many people like Zhong Wei regard with great interest 3their hometowns have changed. Perhaps large hospitals and new roads have appeared. In many places, the government has also built 4and sent teachers from the cities to help.
ZhongWei thinks such development are very good, and these changes will take place in his hometown because some things need to change in order to become 5.
He also thinks among all the changes, some things are6to change, such as the soft and sweet memories of the hometown in people's hearts. That is why people try to find time to go back to visit their hometowns though they are very busy.
恒成立，则实数a的取值范围是 （ ）