Possibility 2: a5-g3
White plays 1.a5-g3.
He has now 2 opportunities to form a row with his next move: 2.a3-d3;xf6
and e9-e4;xh2. In both cases he will capture a piece and there's
nothing Black can do.
Any other move would have left Black a possibility to block White.
Check it out in the problem-diagram:
If White: a3-d3 then Black: i1-g3
If White: c6 then Black: c2
If White: e2 then Black: f1-d2
If White: e9-e4 then Black: e9-e2
If White: f1-f3 then Black: g1-d3