Test center: Choose the appropriate measurement unit according to the situation: unit analysis of length, area and volume: According to life experience and understanding of length unit and data size, we can know that the long-term measurement of mathematics textbooks is based on decimeter. The length of the math textbook is about 2 decimeters; Therefore, choose: C. Comments: This question examines the choice of appropriate measurement units according to the situation, and should pay attention to the actual life, measurement units and data size, and choose flexibly.