Task: Question Generation
Input (English): Passage: The core of the site covers about 6.5 square kilometres (2.5 sq mi) and includes remains of some 70 monumental structures positioned around a dozen plazas. Takalik Abaj has 2 ballcourts and over 239 known stone monuments, including impressive stelae and altars. The granite used to make monuments in Olmec and early Maya styles is much different from the soft limestone used in the Petén cities. The site is also noted for its hydraulic systems, including a temazcal or sauna bath with a subterranean drainage, and Preclassic tombs found in excavations from the late 1990s onwards by Drs. Marion Popenoe de Hatch, Christa Schieber de Lavarreda and Miguel Orrego, from the Ministerio de Cultura y Deportes.
The structures at Takalik Abaj are spread among four groups; the Central, North and West Groups are clustered together but the South Group is located about 5 km (3.1 mi) to the south. The site is naturally defensible, being bordered by steep ravines. The site is spread over a series of nine terraces, which vary in width from 140 to 220 metres (460 to 720 ft) and have faces varying in height from 4.6 to 9.4 metres (15 to 31 ft). These terraces are not uniformly oriented, instead the direction of their retaining faces depends upon the lie of the local terrain. The three main terraces supporting the city are artificial, with over 10 metres (33 ft) of fill being used in places.When Takalik Abaj was at its greatest extent, major architecture in the city covered an area of approximately 2 by 4 kilometres (1.2 by 2.5 mi), although the area occupied by residential construction has not been determined.
The Central Group occupies Terraces 1 to 5, which were artificially levelled. The group contains 39 structures arranged around plazas that are open on the north and south sides. The Central Group was first occupied in the Middle Preclassic and contains a concentration of more than 100 stone monuments.
The West Group consists of 21 structures on Terrace 6, which was also artificially levelled. The structures are arranged around plazas that were left open on the east side. Seven monuments have been found in this group. The West Group is bordered by the rivers Nima on the west and the San Isidro on the east. A notable find in the West Group was the discovery of some jade masks there. The West Group was occupied from the Late Preclassic through to at least the Late Classic.
Output (English): What is the name of the group that contains 21 structures?