The inevitability of China's sustainable development-the grim reality facing China: ① huge population pressure; The shortage of resources is worrying: less per capita, low utilization rate of resources, unreasonable production activities and consumption patterns, and excessive consumption growth; Profound environmental crisis: environmental pollution is still developing and spreading rapidly from cities to rural areas; The scope of ecological damage is expanding and the degree is increasing. The biggest obstacle to China's sustainable development is its huge population. So choose option a in this question.