user: Question: When BMW ActiveE is the vehicle type what is the u.s national average electric mix?
Columns: ['Vehicle', 'Operating mode', 'EPA rated All-electric range', 'EPA rated combined fuel economy', 'Clean electric grid California (San Francisco)', 'U.S. national average electric mix', 'Dirty electric grid Rocky Mountains (Denver)']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Mitsubishi i-MiEV', 'All-electric', 'mi (km)', '112 mpg-e (30kW-hrs/100mi)', '100 g/mi (62 g/km)', '200 g/mi (124 g/km)', '290 g/mi (180 g/km)']
SQL:

assistant: SELECT "U.S. national average electric mix" FROM "Table" WHERE "Vehicle" = 'BMW ActiveE';