A city is like a huge sponge. The labor force is like flowing water between urban and rural areas. When the economy is in an upward period, economic expansion will absorb more labor: When the economy is in a downward period, the economic recession will squeeze these labor forces back into the countryside. The sponge effect caused by China's urban-rural dual structure played a role as an employment buffer in the financial crisis. However, this sponge effect is unsustainable because one of China's basic national conditions is that agricultural surplus labor must be transferred to cities. Transfer. What does the text is intended to explain? ["The economic downturn has affected China's urbanization process.", 'Urban-rural dual structure weakens the impact of economic recession.', "Don't rely too much on the employment buffer effect of the sponge effect.", 'Rural labor should not be victimized by the economic recession.']