How many types of valves were used on this engine that was built on 1902-05?
CREATE TABLE "engine_num7002" (
  "class" text,
  "firebox" text,
  "cylinder_size" text,
  "valves" text,
  "valve_gear" text,
  "number_built" text,
  "years_built" text
);
SELECT COUNT("valves") FROM "engine_num7002" WHERE "years_built"='1902-05';