What is the prime mover for a build date in 1975?
CREATE TABLE "4_axle_units" (
  "model" text,
  "build_date" text,
  "total_produced" real,
  "wheel_arrangement" text,
  "prime_mover" text,
  "power_output" text
);
SELECT "prime_mover" FROM "4_axle_units" WHERE "build_date"='1975';