context: With the collapse of Roman rule in the early 5th century, London ceased to be a capital and the walled city of Londinium was effectively abandoned, although Roman civilisation continued in the St Martin-in-the-Fields area until around 450. From around 500, an Anglo-Saxon settlement known as Lundenwic developed in the same area, slightly to the west of the old Roman city. By about 680, it had revived sufficiently to become a major port, although there is little evidence of large-scale production of goods. From the 820s the town declined because of repeated Viking invasions. There are three recorded Viking assaults on London; two of which were successful in 851 and 886 AD, although they were defeated during the attack of 994 AD.
question: వైకింగ్లు లండన్పై ఎన్ని దాడులు చేశారు?
answers: three