dipiete answer of june 2012 question 1

DC51ACBDBDDBAC
DC52ABCDABACDC
DC53BBBABCBCAB
DC54AACDBCABBD
DC55ACBDBBBABDDC56CDCBCDABAA
DC57CBADABCDAB
DC58CBDBACDBAC
DC59CCBACDCDAA
DC60DBBCDDCDBA
DC61BBCABADCDB
DC62CCACDBDBCA
DC63CBBDDCBBBC
DC65ABDDCAABAA
DC66BBADBBABBD
DC67DCCCCADACD
DC68ADCADAACCD
DC69AABABDCDCA
DC70BCBABDABDC
DC71BABDBBABBB
DC99BCBBAABDAA