已知圆的半径为2，圆心在x轴的正半轴上，且圆与直线3x+4y+4=0相切，则圆的标准方程是 ______．

Peter ________ under the pressure of his position and was unable to work for a year.[ ]

A．settled down  

B．broke down

C．passed away  

D．broke out