The ratio of the work done by the machine ______ the work done on it is called the efficiency of the machine.

A．the way you handle major events may cause stress
B．what should be done to avoid stress
C．what kind of event would cause stress
D．how to cope with sudden changes in life

A．how much pressure you are under
B．how positive events can change your life
C．how stressful a major event can be
D．how you can deal with life-changing events