Question: Which battle to which Æthelwulf of Berkshire lost his life was a battle on 4 January 871 at Reading in what is now the English county of Berkshire?

full_context: Document 1: Battle of Reading (871): The first Battle of Reading was a battle on 4 January 871 at Reading in what is now the English county of Berkshire. It was one of a series of battles, with honours to both sides, that took place following an invasion of the then kingdom of Wessex by an army of Danes led by Bagsecg and Halfdan Ragnarsson in an attempt to conquer Wessex. Both battle and campaign are described in the "Anglo-Saxon Chronicle", and this account provides the earliest known written record of the existence of the town of Reading.
Document 2: Battle of Marton: The Battle of Marton or Meretum took place on approximately 22 March 871 at a place recorded as Marton, perhaps in Wiltshire or Dorset. According to the Anglo-Saxon Chronicle, the battle took place "about two months" after a prior loss to the Danes at the Battle of Basing, in which both sides suffered heavy losses. King Æthelred of Wessex and Prince Alfred fought against the Danes, who were possibly under the leadership of King Halfdan Ragnarsson, who had commanded many of the earlier battles that year. The Chronicle tells us that the forces were split in two and that the Danes were initially put to flight, but at the end of the day and after "much slaughter" the Danes remained masters of the field. Among the fallen was Bishop Heahmund.
Document 3: Battle of Englefield: The Battle of Englefield was a battle on 31 December 870 at Englefield, near Reading in what is now the English county of Berkshire. It was one of a series of battles, with honours to both sides, that took place following an invasion of the then kingdom of Wessex by an army of Danes, during which the Danes had established a camp at Reading. Both battle and campaign are described in the "Anglo-Saxon Chronicle".
Document 4: Æthelwulf of Berkshire: Æthelwulf of Berkshire (before 825 – 4 January, 871) was a Saxon ealdorman. In 860 he and other men of Berkshire fought off a band of pirates near Winchester, Hampshire. Later he mustered a force of 1400 men against an army of Danes, won the 31 December 870 Battle of Englefield on behalf of the then kingdom of Wessex. He received a land grant in 843/44 from Brihtwulf, king of Mercia; and lost his life at the Battle of Reading.
Document 5: Battle of Ashdown: The Battle of Ashdown, in Berkshire (possibly the part now in Oxfordshire), took place on 8 January 871. Alfred the Great, then a prince of only 22, led the army of his brother, King Ethelred of Wessex, in a victorious battle against the invading Danes.
Document 6: Beorhtwulf of Mercia: Beorhtwulf (] , meaning "bright wolf"; also spelled "Berhtwulf"; died 852) was King of Mercia, a kingdom of Anglo-Saxon England, from 839 or 840 to 852. His ancestry is unknown, though he may have been connected to Beornwulf, who ruled Mercia in the 820s. Almost no coins were issued by Beorhtwulf's predecessor, Wiglaf, but a Mercian coinage was restarted by Beorhtwulf early in his reign, initially with strong similarities to the coins of Æthelwulf of Wessex, and later with independent designs. The Vikings attacked within a year or two of Beorhtwulf's accession: the province of Lindsey was raided in 841, and London, a key centre of Mercian commerce, was attacked the following year. Another Viking assault on London in 851 "put Beorhtwulf to flight", according to the "Anglo-Saxon Chronicle"; the Vikings were subsequently defeated by Æthelwulf. This raid may have had a significant economic impact on Mercia, as London coinage is much reduced after 851.
Document 7: Battle of Assandun: The Battle of Assandun (or Essendune) was fought between Danish and English armies on 18 October 1016. There is disagreement whether Assandun may be Ashdon near Saffron Walden in north Essex or, as long supposed, Ashingdon near Rochford in southeast Essex, England. It ended in victory for the Danes, led by Canute the Great, who triumphed over the English army led by King Edmund Ironside. The battle was the conclusion to the Danish reconquest of England.
Document 8: Battle of Ashdown: The Battle of Ashdown, in Berkshire (possibly the part now in Oxfordshire), took place on 8 January 871. Alfred the Great, then a prince of only 22, led the army of his brother, King Ethelred of Wessex, in a victorious battle against the invading Danes.
Document 9: Bagsecg: Bagsecg (also known as Bægsecg or Bagsec) (died 8 January 871) was a Viking leader commonly referred to as a king. In 870 or 871 he led The Great Summer Army to England. His forces joined those of the Great Danish Army which had already overrun much of England. He and Halfdan Ragnarsson became the leaders of a joint invasion of the Kingdom of Wessex. He was killed at the Battle of Ashdown, fighting a West Saxon Army led by King Ethelred and his younger brother, the future Alfred the Great.
Document 10: Æthelwulf: Æthelwulf (Old English for "Noble Wolf"; died 13 January 858) was King of Wessex from 839 to 858. In 825, his father, King Egbert, defeated King Beornwulf of Mercia, ending a long Mercian dominance over Anglo-Saxon England south of the Humber. Egbert sent Æthelwulf with an army to Kent, where he expelled the Mercian sub-king and was himself appointed sub-king. After 830, Egbert maintained good relations with Mercia, and this was continued by Æthelwulf when he became king in 839, the first son to succeed his father as West Saxon king since 641.
Document 11: Great Heathen Army: The Great Viking Army or Great Danish Army, known by the Anglo-Saxons as the Great Heathen Army (OE: "mycel hæþen here"), was a coalition of Norse warriors, originating from Denmark (and likely also from Sweden and Norway) who came together under a unified command to invade the four Anglo-Saxon kingdoms that constituted England in AD 865.
Document 12: Battle of Basing: The Battle of Basing was a battle on 22 January 871 at Old Basing in what is now the English county of Hampshire. It was one of a series of battles that took place following an invasion of the then kingdom of Wessex by an army of Danes. Both battle and campaign are described in the "Anglo-Saxon Chronicle".
Document 13: Battle of Aclea: The Battle of Aclea occurred in 851 between the West Saxons led by Æthelwulf, King of Wessex and the Danish Vikings. It resulted in a West Saxon victory.
Document 14: History of Herefordshire: The History of Herefordshire starts with a shire in the time of Athelstan (895–939), and Herefordshire is mentioned in the "Anglo-Saxon Chronicle" in 1051. The first Anglo-Saxon settlers, the Magonsætan, were a sub-tribal unit of the Hwicce who occupied the Severn valley. The Magonsætan were said to be in the intervening lands between the Rivers Wye and Severn. The undulating hills of marl clay were surrounded by the Welsh mountains to the west; the Malvern Hills to the east; the Clent Hills of the Shropshire borders to the north, and the indeterminate extent of the Forest of Dean to the south. The shire name first recorded in the "Anglo-Saxon Chronicle" was derived from "Here-ford", Old English for "Army crossing", the location for the city. The area was covered first by Offa of Mercia, who constructed the dyke as a boundary to keep warring tribes out of the Mercian kingdom: an early indication of the ambivalent relations with the Welsh. The shire as an administrative unit was developed from Alfred the Great's Burghal Hidage, and the Shire-reeve courts of the Hundred. King Edgar the Ætheling was a regular visitor, and founded the diocese, previously part of the see of Worcester, in 976. The establishment of a centre of law and justice was supported by a monastic chapter that flourished during the Tenth century Reformation. Hereford's geographical location at the hub of the shire allowed Anglo-Saxon ealdormen to manage affairs; and Hereford played a vital role in the Scandinavian wars until Ralph, Earl Hereford was deposed by the regal Earl Harold Godwinson.
Document 15: Halfdan Ragnarsson: Halfdan Ragnarsson (Old Norse: "Hálfdan" ; or "Healfdene"; Old Irish: "Albann" ; died 877) was a Viking leader and a commander of the Great Heathen Army which invaded the Anglo-Saxon kingdoms of England, starting in 865. According to the tradition recorded in the Norse sagas he was one of the sons of Ragnar Lodbrok, and his brothers included Björn Ironside, Ivar the Boneless, Sigurd Snake-in-the-Eye and Ubba. He was the first Viking King of Northumbria and a pretender to the throne of Kingdom of Dublin. He died at the Battle of Strangford Lough in 877 trying to press his Irish claim.
ground_truth: Battle of Reading