For the year 1975, what is Propulsion, with a Number (quantity ordered) of 4756-4788 (33 buses)?
CREATE TABLE "retired" (
  "make_model" text,
  "length" text,
  "year" real,
  "numbers_quantity_ordered" text,
  "fuel_propulsion" text,
  "year_retired" text
);
SELECT "fuel_propulsion" FROM "retired" WHERE "year"=1975 AND "numbers_quantity_ordered"='4756-4788 (33 buses)';