Database: toxicology
Question: On average how many carcinogenic molecules are single bonded?
SQL: SELECT AVG(single_bond_count) FROM (SELECT T3.molecule_id, COUNT(T1.bond_type) AS single_bond_count FROM bond AS T1  INNER JOIN atom AS T2 ON T1.molecule_id = T2.molecule_id INNER JOIN molecule AS T3 ON T3.molecule_id = T2.molecule_id WHERE T1.bond_type = '-' AND T3.label = '+' GROUP BY T3.molecule_id) AS subquery