What is the highest gender ratio of the residential areas in Arecibo county?
"ARECIBO" is the county; highest gender ration refers to Max(Divide (male_population, female_population))
SELECT CAST(T1.male_population AS REAL) / T1.female_population FROM zip_data AS T1 INNER JOIN country AS T2 ON T1.zip_code = T2.zip_code WHERE T2.county = 'ARECIBO' AND T1.female_population <> 0 ORDER BY 1 DESC LIMIT 1