Jane is          high school student in the United States.

A．an 18-year-old

B．a 18-years-old

C．an 18-years-old

D．a 18-year-old

电流表用以指示蓄电池充电或放电的电流，充电时，指针偏向"+"号一侧；放电时，指针偏向"-"号一侧。