请写出下列名词的复数形式
1 pen ( )
2 class ( )
3 box ( )
4 ruler ( )
5 teacher ( )
6 name ( )
7 number ( )
8 year ( )
9 eraser ( )
10 map ( )
11 apple ( )
12 toy ( )
13 desk ( )
14 book ( )
15 cake ( )
16 egg ( )
17 bus ( )
18 blackboaed ( )