1. Biological habit: Piranhas mainly live in the Amazon River basin of South America, and have specific requirements for water temperature, water quality and food. In China, piranhas can't survive and reproduce because of different climatic conditions and ecological environment.
2. Ecological environment: The climate difference between China and South America is quite large, which makes piranhas unable to adapt to the living conditions in China. In addition, many rivers, lakes and marine ecosystems in China have no original populations of piranhas, which prevents them from flooding in China.
3. Human activities: Because piranhas have specific requirements for water temperature, human activities such as the construction of hydropower stations and the change of river courses may affect the living environment of piranhas, thus limiting their spread.
4. Entry restrictions: China has strict import and export regulations on animals and plants to prevent the invasion of pests. If piranhas intend to enter China illegally, they will be intercepted by customs investigation.
5. Civic awareness: The public in China has a high awareness of biological protection. If piranhas are found illegally released, they will immediately report to the relevant departments, which will also help prevent piranhas from flooding.
To sum up, piranhas can't flood in China due to biological habits, ecological environment, human activities, entry restrictions, civic awareness and other factors.