CREATE TABLE table_3904 ("Standard" TEXT, "Date" TEXT, "CO  ( g/kWh ) " TEXT, "NO x  ( g/kWh ) " TEXT, "HC  ( g/kWh ) " TEXT, "PM  ( g/kWh ) " TEXT)

What's the PM for the standard with 12.3 g/kWh CO?
SELECT "PM (g/kWh)" FROM table_3904 WHERE "CO (g/kWh)" = '12.3'
SELECT "PM (g/kWh)" FROM "table_3904" WHERE "CO (g/kWh)" = '12.3'