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.
单项选择题 The Mona Lisa, ______ in Italy, is now in the Louvre, a museum in Paris.
单项选择题 The bird flew upward and dropped the shellfish onto the rock to ______ it open.
