According to the air traffic rules, you ______ switch off your mobile phone before boarding.
A:ought to
B:may
C:can
D:would
具有涵养水源、保持水土功能,有“绿色水库”之称的生态系统是( )
函数fx=Asinωx+φA>0,ω>0,φ<π2的部分图象如图所示,则函数y=fx对应的解析式为( )