by **scrose** » Tue Jul 05, 2005 2:11 pm

In cells r2c1 and r9c1 the only candidates are 6 and 7. One of these cells must be a 6 and the remaining cell must be a 7. Therefore no other cells in column 1 can contain a 6 or a 7, so you can safely eliminate the candidate 6's and 7's from cells r1c1 and r3c1. These eliminations will let you fill a cell in block 2.