?????

?????????ABC???????????????????????????
???????????????????????????????????????????,?????

1?A??
??A?????????????A???????A?A??????????????????A??????????????

2?B????
????B2?B??????????B??????????(?B2)(?B1 ?)

3?C??
?C???????C?????????C???????????????

4???
????????????????????????

5???
?????????????????????????2????

??

?

???????????????B1???B1???????????????????????????????

??

?????????????? ?????????????????

?????
1.????
2.?
3.???
4.??


??

????
???????????????...