what are the right options

a) The signature bit for memory and cache data are equal is . . . . . . . . . . . . . . (valid / dirty). b) The policy name of immediately write when cache data is changed . . . . . . . . . . . . . . . . . . . . . . . . . . . . (Write through / Write back) c) The decision for new written data for memory is also keep in cache . . . . . . . . . . . . . . . . . . . . . . . . . . . . (is always better / is well sometimes/ is never useful).