A:利于灌溉`
B:减少蒸发
C:利用地势自流
D:节约成本
We should give the children with ____ ____ a chance to show their skills.
What time ____ (do) your mom usually get up?
设非零向量a→,b→,c→,满足a→=b→=c→,a→+b→=c→,则sin<a→,b→>= ( )