Table schema:
CREATE TABLE table_29289372_1 (area__km²_ VARCHAR, income_classification VARCHAR, population__2010_ VARCHAR)

Question: When 318676 is the population of 2010 and 1st class is the income classification what is the area in kilometers squared?
SQL: SELECT area__km²_ FROM table_29289372_1 WHERE income_classification = "1st Class" AND population__2010_ = 318676