_______是北京最古老、最著名的繁华街市。也是北京第一条旅游步行街。
参考答案：大栅栏商业街
You never know_____happiness will turn up next, so grasp every chance around you. [ ]
A. when
B. where
C. why
D. how
水泥是一种气硬性无机胶凝材料。( )
