Definition: In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.
Input: Passage: Altar 13 is another early Maya monument dating to the Late Preclassic. Like Altar 12 it was probably originally erected as a vertical stela. At some point it was deliberately broken, with severe damage inflicted upon the main portion of the sculpture, obliterating the central and lower portions. At a later date it was reused as a horizontal altar. The remains of two figures can be seen flanking the damage lower portion of the monument and the large head of the sacred bird survives above the area of damage. The right hand figure is wearing an interwoven skirt and is probably female.Altar 18 was one of five monuments forming a north-south row at the base of Structure 8 on Terrace 3.Altar 28 is located near Structure 10 in the Central Group. It is a circular basalt altar just over 2 metres (79 in) in diameter and 0.5 metres (20 in) thick. On the front rim of the altar is a carving of a skull. On the upper surface are two relief carvings of human feet.Altar 30 is embedded in the fourth step of the access stairway to Terrace 3 in the Central Group. It has four low legs supporting it and is similar to Altar 9.Altar 48 is a very early example of the Early Maya style of sculpture, dating to the first part of the Late Preclassic, between 400 and 200 BC. Altar 48 is fashioned from andesite and measures 1.43 by 1.26 metres (4.7 by 4.1 ft) and is 0.53 metres (1.7 ft) thick. It is located near the southern extreme of Terrace 3, where it is one of a row of 5 monuments running east-west. It is carved on its upper face and upon all four sides. The upper surface bears the intricate design of a crocodile with its body in the form of a symbol representing a cave and containing the figure of a seated Maya wearing a loincloth. The sides of the monument are carved with an early form of Maya hieroglyphs, the text appears to refer directly to the person depicted on the upper surface. Altar 48 had been carefully covered by Stela 14. The emergence of a Maya ruler from the body of the crocodile parallels the myth of the birth of the Maya maize god, who emerges from the shell of a turtle. As such, Altar 48 may be one of the earliest depictions of Maya mythology used for political ends.
Output:
What structure has two relief carvings of human feet?