Who won the womens singles when Marc Zwiebler won the men's singles?
CREATE TABLE "21st_century" (
  "year" real,
  "mens_singles" text,
  "womens_singles" text,
  "mens_doubles" text,
  "womens_doubles" text,
  "mixed_doubles" text
);
SELECT "womens_singles" FROM "21st_century" WHERE "mens_singles"='Marc Zwiebler';