Read this and answer the question. If the question is unanswerable, say "unanswerable".

From the 2011 Census, the Office for National Statistics published that Plymouth's unitary authority area population was 256,384; 15,664 more people than that of the last census from 2001, which indicated that Plymouth had a population of 240,720. The Plymouth urban area had a population of 260,203 in 2011 (the urban sprawl which extends outside the authority's boundaries). The city's average household size was 2.3 persons. At the time of the 2011 UK census, the ethnic composition of Plymouth's population was 96.2% White (of 92.9% was White British), with the largest minority ethnic group being Chinese at 0.5%. The white Irish ethnic group saw the largest decline in its share of the population since the 2001 Census (-24%), while the Other Asian and Black African had the largest increases (360% and 351% respectively). This excludes the two new ethnic groups added to the 2011 census of Gypsy or Irish Traveller and Arab. The population rose rapidly during the second half of the 19th century, but declined by over 1.6% from 1931 to 1951.

What was the population of Plymouth unitary authority circa 2011?
256,384