Among the car models introduced in the market in 1970, how many of them have the USA as their origin country?
introduced in the market in 1970 refers to model_year = 1970; have the USA as origin country refers to country = 'USA'
SELECT COUNT(*) FROM production AS T1 INNER JOIN country AS T2 ON T1.country = T2.origin WHERE T1.model_year = 1970 AND T2.country = 'USA'