user: Question: What is the average birth rate that has a total fertility rate of na, and a natural growth smaller than 2.5?
Columns: ['Year', 'Births (000s)', 'Deaths', 'Natural Growth', 'Total Fertility Rate']
Types: ['text', 'real', 'real', 'real', 'text']
Sample row: ['1990', 0.7, 0.4, 0.3, '1.58']
SQL:

assistant: SELECT AVG("Births (000s)") FROM "Table" WHERE "Total Fertility Rate" = 'na' AND "Natural Growth" < 2.5;