What is the average birth rate that has a total fertility rate of na, and a natural growth smaller than 2.5?
CREATE TABLE table_66703 (
    "Year" text,
    "Births (000s)" real,
    "Deaths" real,
    "Natural Growth" real,
    "Total Fertility Rate" text
)
SELECT AVG("Births (000s)") FROM table_66703 WHERE "Total Fertility Rate" = 'na' AND "Natural Growth" < '2.5'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: What is the average birth rate that has a total fertility rate of na, and a natural growth smaller than 2.5? ### Input: CREATE TABLE table_66703 (
    "Year" text,
    "Births (000s)" real,
    "Deaths" real,
    "Natural Growth" real,
    "Total Fertility Rate" text
) ### Response: SELECT AVG("Births (000s)") FROM table_66703 WHERE "Total Fertility Rate" = 'na' AND "Natural Growth" < '2.5'