??
?2?????16???? ????????????????,??,2????16?20???????17????20??,??????????????????????????????????4~6??????????????????????7~8?????????;???????????????????????????????????7?????????????????????????8???
???????:
1???????????????????????,??????????;
2?????????????????,?????????????????????;
3?????????,????????????????????;
4??????,??????
????:???????????????
???????????:7?? ?????????7?8? ?????????????