一道几何题.快如图,∠AOC=90°,ON是锐角∠COD的角平分线,OM是∠AOD的角平分线,那么∠MON=?A．45°B．90°C． ∠AODD． ∠AOD+45°

一道几何题.快
如图,∠AOC=90°,ON是锐角∠COD的角平分线,OM是∠AOD的角平分线,那么∠MON=?
A．45°
B．90°
C． ∠AOD
D． ∠AOD+45°

∠MON=∠AON-∠AOM
=∠AOC+∠CON-(1/2)∠AOD
=90+(1/2)∠COD-(1/2)(∠AOC+∠COD)
=90+(1/2)∠COD-(1/2)*90-(1/2)∠COD
=90-45
=45度
选A