What format is the link for the polling data for February 10–28, 2011?
CREATE TABLE "opinion_polls" (
  "polling_firm" text,
  "date_of_polling" text,
  "link" text,
  "progressive_conservative" real,
  "liberal" real,
  "new_democratic" real
);
SELECT "link" FROM "opinion_polls" WHERE "date_of_polling"='February 10–28, 2011';