How many items were listed under world rank under the nation of Nigeria?
CREATE TABLE "list_of_african_countries_by_gdp_per_cap" (
  "african_rank" real,
  "world_rank" real,
  "nation" text,
  "gdp_per_capita" text,
  "total_gdp_world_rank" real
);
SELECT COUNT("world_rank") FROM "list_of_african_countries_by_gdp_per_cap" WHERE "nation"='Nigeria';