1. Her work has __________(促成) enormously to our understanding of this difficult subject.
2. This knife needs __________(使变锋利).
3. We should value our __________ (宝贵的) friendship.
4. The government managed to __________ (维持) prices.
5. The panda is one of the __________ (濒危的) animals.
6. Efforts to __________ (保护) the peace have failed.
