从下列各组单词中选出一个发音与众不同,将其序号填入括号中
1.a.cup b.bus c.toothbrush d.music
2.a.yellow b.how c.snow d.window
3.a.Thusday b.thank c.math d.clothes
4.a.kite b.pig c.ice-cream d.bike
5.a.docor b.horse c.short d.fork
6.a.why b.who c.whose d.whom
还有，Don't open the door to strangers.