什么是建筑材料，怎样进行分类？

A. Create a new government.
B. Make its citizens happier.
C. Make the world people happy.
D. Create a new office in the government.

A. The government doesn't do anything to make people happy.
B. The government doesn't care whether people are happy or not.
C. The government hasn't created an office to study people's happiness.
D. The government hasn't solved many problems that make people unhappy.