What is the average ROM size of a phone made by company Nokia?
SELECT avg ( T1.ROM_MiB )  FROM chip_model AS T1 JOIN phone AS T2 ON T1.Model_name   =   T2.chip_model WHERE T2.Company_name  =  "Nokia Corporation"
average ROM size refers to AVG(ROM_MiB); phone made by company Nokia refers to WHERE Company_name = 'Nokia Corporation';