Database: yelp
Question: How many people reviewed " Bistro Di Napoli " in 2015 ?
SQL:
SELECT COUNT ( DISTINCT t3.name ) FROM review AS t2 JOIN business AS t1 ON t2.business_id  =  t1.business_id JOIN USER AS t3 ON t3.user_id  =  t2.user_id WHERE t1.name  =  "Bistro Di Napoli" AND t2.year  =  2015;