道岔尖轨、可动心轨伤损或病害达到什么状况应及时修理或更换？

合理的交货地点是指可以减少企业的运输与装卸作业。( )

a=3是直线ax+2y+3a=0和直线3x+（a-1）y=a-7平行且不重合的______条件．

Mes frères aiment parler à des personnes______ ils ne connaissent pas.

Helen had to shout  _____ above the sound of the music. [ ]

A. to make herself hear

B. making herself hear

C. to make herself heard

D. making herself heard

构造应力场