变直径有压圆管的流动，上游断面1的直径d1=150mm，下游断面2的直径d2=300mm，断面1的平均流速v1=6m／s，断面2的平均流速v2为：（）

A.3m／s

B.2m／s

C.1.5m／s

D.1m／S

Rose is too young to go to school. [ ]

A. old enough to go to school

B. too short to go home

C. so young that she can't go to school

D. a school girl