单项选择题
Effective prevention against physical harms has never been ______ urgently needed, especially in schools.
单项选择题 People who walk on the grass are ______ to a fine of $5.
单项选择题 Dr. Hawking has made much contribution to the theories of modem physics at the ______ of his health.
单项选择题 John Smith, being a diligent student, never refuses to ______ more responsibilities that are assigned to him.
