Database: toxicology
Question: What type of bond is there between the atoms TR004_8 and TR004_20?
SQL: SELECT T1.bond_type FROM bond AS T1 INNER JOIN connected AS T2 ON T1.bond_id = T2.bond_id WHERE T2.atom_id = 'TR004_8' AND T2.atom_id2 = 'TR004_20' OR T2.atom_id2 = 'TR004_8' AND T2.atom_id = 'TR004_20'