Think the Kentucky Derby is a tough puzzle to unlock? Try wrapping your brain around the two open allowance races on Arlington Park&rsquo;s Derby Day card.\r\nRace 5, a second-level, six-furlong Polytrack allowance also open to $40,000 claimers, drew a field of eight fillies and mares, and there&rsquo;s really not an easy toss among the entire group. The octet made their most recent starts at five different venues, a couple &ndash; Heart Thief and Rose Mountain &ndash; already sport wins under these conditions, and six of the horses can boast of a previous win over Arlington&rsquo;s synthetic surface. Ripe Tomato, a longtime statebred sprint-stakes denizen, has accomplished the most but was dull last out at Hawthorne and returns on short rest for a $40,000 tag. Rattlin Gypsy has the strongest recent form, with three wins in her last four starts.\r\nIt&rsquo;s possible that race 8, for 3-year-old entry-level allowance grass runners, could produce a starter or two for the May Arlington Classic, first leg of Arlington&rsquo;s Mid-America Triple, but most of the top races turned in by the better-looking entrants in this nine-horse field came in claiming races. Live In Joy, Crown of Aragorn, and Terminal One all won or placed over the Arlington grass course last season.