Among the cars produced in year 1973, how many of the cars have horsepower less than 100?
produced in year 1973 refers to model_year = 1973; have horsepower less than 100 refers to horsepower < 100
SELECT COUNT(*) FROM data AS T1 INNER JOIN production AS T2 ON T1.ID = T2.ID WHERE T2.model_year = 1973 AND T1.horsepower < 100