| abc |
| ac+bc |
| 1 |
| 3 |
| abc |
| ab+ac |
| 1 |
| 4 |
| abc |
| bc+ab |
| 1 |
| 5 |
则ac+bc=3abc①，ab+ac=4abc②，bc+ab=5abc③，
①+②+③得，2（ab+bc+ca）=12abc，
即
| abc |
| ab+bc+ca |
| 2 |
| 12 |
| 1 |
| 6 |
故选A．
| ab |
| a+b |
| 1 |
| 3 |
| bc |
| b+c |
| 1 |
| 4 |
| ca |
| c+a |
| 1 |
| 5 |
| abc |
| ab+bc+ca |
| 1 |
| 6 |
| 1 |
| 12 |
| 2 |
| 15 |
| 1 |
| 20 |
| abc |
| ac+bc |
| 1 |
| 3 |
| abc |
| ab+ac |
| 1 |
| 4 |
| abc |
| bc+ab |
| 1 |
| 5 |
| abc |
| ab+bc+ca |
| 2 |
| 12 |
| 1 |
| 6 |