## 3 Results  
### Time Series Characteristics of the CPC  
At present, the production cost of food and other agricultural products is primarily defined from the perspective of the economic and accounting cost in Western economics. In the most dependable _Compilation of National Agricultural Product Costs and Benefits_ in China, the cost of agricultural products is largely concentrated in the field of production. However, research concerned with the circulation cost of agricultural products at the national level lacks necessary data support, making the transaction costs are difficult to measure. Therefore, in order to make complete use of the data at the national level, this paper follows relevant research results [33; 34] and conducts quantitative analysis primarily from the perspective of accounting costs. The CPC is taken to be the total consumption of production factors per unit of corn output, including material and service costs, labor costs, and land costs. Figure 1 shows the CPC of the country and the three major corn producing areas for the period from 2008 to 2018. The results show that the CPC in the southern mountainous and hilly area is much higher than that in both the Huang-huai-hai summer corn area and the northern spring corn area. Furthermore, the production cost of the northern spring corn area is slightly higher than that in the Huang-huai-hai summer corn area. The above results are largely the result of the tense human-land relationship in the corn area of the southern mountainous and hilly area [35]. The proportion of mountainous and hilly areas in Yunnan, Guizhou, Sichuan, Guangxi, and other provinces all exceeds 90%. Moreover, the degree of land fragmentation is high, which results in higher labor input level. Noticeably, the CPC is most significantly affected by labor costs [34; 35]. Therefore, the CPC in the southern mountainous and hilly area is higher than in other regions. From 2008 to 2018, the CPC in the three main producing areas manage to maintain an upward trend. However, it fluctuates greatly. The upward trend can be roughly divided into three stages: 2008-2011 as a period of rapid growth, 2012-2014 as a period of relative stability, and 2015-2018 as a period of volatile growth. Firstly, the period from 2008 to 2011 is one of rapid growth in corn production cost. During this period, the CPC increases rapidly. The cost in the southern mountain and hilly corn area rises rapidly from 1982.23 CNY/ton (311.21 USD/ton) to 2640.42 CNY/ton (414.55 USD/ton) in 2011, with an average annual growth rate of 10.03%. Furthermore, the cost in northern spring corn area increases from 1473.72 CNY/ton (231.37 USD/ton) to 1748.27 CNY/ton (274.48 USD/ton), which is a cumulative increase of 274.55 CNY/ton (43.10 USD/ton), with an average annual growth rate of 5.86%. Secondly, the period between 2012 and 2014 is taken to be a relatively stable period with regards to the CPC. During this period, the overall CPC is relatively stable. From 2012 to 2014, the cost per ton in the Southern mountainous and hilly area is 2587.39 (406.22), 2628.97 (412.75), and CNY 2658.59 (USD 417.40). The rate of change in comparison to the previous year is 1.62%. Furthermore, the average annual growth rate of the CPC in the northern spring corn area in the three years does not exceed 2.00%. The change rate of cost compared with the previous year is 2.60% in the Huang-huai-hai summer corn area. Thirdly, the period from 2015 to 2018 is one of irregular growth with regards to the CPC. From 2015 to 2016, the CPC in three major producing regions declines. However, it begins to increase after 2017. For example, the CPC per ton in the southern mountainous and hilly area is CNY 1992.84 (USD 312.88) in 2017, while the cost increases to CNY 2120.80 (USD 332.97) in 2018. This situation may be associated with the lack of available data after 2018. The CPC shows a fluctuating upward trend during the research periods.