user: ### NOW ANSWER:
Question: What is average SiO 2, when Na 2 O is 7.53, and when Al 2 O 3 is less than 2.13?
Columns: ['Objects', 'Date', 'SiO 2', 'Al 2 O 3', 'Fe 2 O 3', 'K 2 O', 'Na 2 O']
Types: ['text', 'text', 'real', 'real', 'real', 'real', 'real']
Sample row: ['Eye bead', '4th to 3rd centuries BC', 55.0, 2.13, 0.62, 4.05, 7.53]
SQL:

assistant: SELECT AVG("SiO 2") FROM "Table" WHERE "Na 2 O" = 7.53 AND "Al 2 O 3" < 2.13;