Jon Corzine had better come up with an explanation for MF Global’s missing millions, and fast, because U.S. lawmakers are losing patience. The Senate Agriculture Committee is holding its second hearing this week in an effort to investigate just how futures firm MF Global collapsed and exactly what happened to all of its customers’ money. Corzine, the firm’s chief, continues to insist he has no idea where the money went, and lawmakers are getting frustrated. “This isn’t the Dark Ages. MF Global didn’t keep their books with feather quills and dusty ledgers. The rules about keeping customer money segregated are pretty straightforward,” said Agriculture Committee chairwoman Debbie Stabenow.