Air pollution has become ______ than ever before. We must do something to stop it.
A:serious
B:more serious
C:most serious
D:the most serious
孝感地区的房屋多为坐北朝南且屋顶坡度大,这体现了( )
如图,MN为⊙O的弦,∠M=50°,则∠MON等于 ____