CREATE TABLE table_3901 ("Standard" TEXT, "Date" TEXT, "CO  ( g/kWh ) " TEXT, "NO x  ( g/kWh ) " TEXT, "HC  ( g/kWh ) " TEXT, "PM  ( g/kWh ) " TEXT)

How many different standards have a PM of 0.02 g/kWh?
SELECT COUNT("Standard") FROM table_3901 WHERE "PM (g/kWh)" = '0.02'
SELECT COUNT("Standard") FROM "table_3901" WHERE "PM (g/kWh)" = '0.02'