Table: tennis_match_stats_8
Columns: date
tournament
surface
opponent_in_the_final
score

Question: What type of surface was used for the carthage tournament on august 14, 1994?
SQL: SELECT "surface" FROM tennis_match_stats_8 WHERE "tournament" = 'carthage' AND "date" = 'august 14, 1994'

Question: Which tournament has marielle bruens for the opponent in the final?
SQL: SELECT "tournament" FROM tennis_match_stats_8 WHERE "opponent_in_the_final" = 'marielle bruens'

Question: What surface was used on may 13, 2007?
SQL: SELECT "surface" FROM tennis_match_stats_8 WHERE "date" = 'may 13, 2007'