CREATE TABLE table_78291 ("Disaster" CLOB, "Year" CLOB, "Countries Affected" CLOB, "Scale of Disaster" CLOB, "People Assisted" CLOB, "Nature of Help" CLOB)

Which year did USA undergo a disaster?
SELECT "Year" FROM table_78291 WHERE "Countries Affected" = 'usa'
SELECT "Year" FROM "table_78291" WHERE "Countries Affected" = 'usa'