把弯曲的河道改直,能够缩短航程,这样做的道理是( )
A:两点之间,射线最短
B:两点确定一条直线
C:两点之间,直线最短
D:两点之间,线段最短
For me, ____ ____ ____ ____ it's a challenge,but ____
____ ____ ____it's a chance to study.