The relative intensity is the ratio between two different but related aggregate indicators, which are used to explain the comprehensive indicator of the intensity, density, and prevalence of the phenomenon. According to the above definition, which of the following is the following description that belongs to the relative number of intensity? ["In 2006, China's per capita GDP exceeded US $ 2,000.", 'In 2007, the average monthly salary of urban employees in a certain area was 1,800 yuan.', 'The ratio of per capita GDP of the United States to China is about 20: 1.', 'The labor productivity in a certain region in January 2008 was 3,000 yuan / person.']