Test Analysis: This question examines the historical facts of China's use of iron farm tools and Niu Geng during the Spring and Autumn Period and the Warring States Period. During the Spring and Autumn Period, iron farm tools and Niu Geng began to appear in China. During the Warring States Period, the use of iron farm tools was expanded and further popularized in Niu Geng. The popularization of iron farm tools and Niu Geng is the development of agriculture from "coupling thousands of households" to "individual farming". So, choose answer D.

Test sites: the new curriculum standard of People's Education Edition, the first volume of seventh grade history, the emergence of the country and social changes, and the era of great changes.