What is the census rating of the city that hosted the competition with the theme "Carnival M is back!"?
SELECT T1.Census_Ranking FROM city AS T1 JOIN farm_competition AS T2 ON T1.City_ID   =   T2.Host_city_ID where T2.Theme  =  "Carnival M is back!"
census rating of the city refers to T1.Census_Ranking; hosted the competition refers to joining city with farm_competition on City_ID = Host_city_ID; competition with theme 'Carnival M is back!' refers to T2.Theme = 'Carnival M is back!'