THEY SCORE, but they're never the primary option. They defend, but their jobs extend beyond locking down a top scorer. They start, but they don't star. Meet the guys commonly referred to as X-factors: players whose performance is often a bellwether of their team's success. These X's exhibit great degrees of variance, blowing up in wins and falling apart in losses. And for a six-pack of top NCAA tourney teams, a deep run may ultimately hinge on whichever version of their X shows up over the next few weeks.