probability(sum of dice) 2

Inthrowing two fair dice, find the probability that the sum of the twonumber is 7.
a：there are：(1，6)，(2，5)，(3，4)，(4，3)，(5，2)，(6,1)
P=6/36=1/6
b：there are：1and6，2and5，3and4
P=3/21=1/7
Which is correct? Why another is incorrect? In what situation should I use 21as all
possible outcomes of throwing two fair dice?
Sol
　｜　１　　２　　３　　４　　５　　６
─＋─────────────────
１｜　２　　３　　４　　５　　６　　７
２｜　３　　４　　５　　６　　７　　８
３｜　４　　５　　６　　７　　８　　９
４｜　５　　６　　７　　８　　９　１０
５｜　６　　７　　８　　９　１０　１１
６｜　７　　８　　９　１０　１１　１２

p=6/36=1/6
a are correct

the total possible outcomes are (1,1),(1,2),(1,3),(1,4),(1,5),(1,6),(2,1),(2,2),...
then I grouped (1,2) and (2,1) as 1 outcome and so on= =, I finally got 21 outcomes
but I realize it's wrong now...

I wonder how you can find 21 possible outcomes when two fair dice are thrown.