Question: What are the names of the dishes with a stable price that were created in April of 2011?
SQL: SELECT T1.name FROM Dish AS T1 INNER JOIN MenuItem AS T2 ON T1.id = T2.dish_id WHERE SUBSTR(T2.created_at, 1, 4) = '2011' AND SUBSTR(T2.created_at, 7, 1) = '4' AND T1.highest_price IS NULL