Questions 19-20 are based on the following.>>
Archaeologists seeking the location of a legendary siege and destruction of a city are excavating in several possible places, including a middle and a lower layer of a large mound. The bottom of the middle layer contains some pieces of pottery of type 3, known to be from a later period than the time of the destruction of the city, but the lower layer does not.>>
20. The force of the evidence cited above is most seriously weakened if which of the following is true?>>
(A) Gerbils, small animals long native to the area, dig large burrows into which objects can fall when the burrows collapse.>>
(B) Pottery of types 1 and 2, found in the lower level, was used in the cities from which, according to the legend, the besieging forces came.>>
(C) Several pieces of stone from a lower-layer wall have been found incorporated into the remains of a building in the middle layer.
(D) Both the middle and the lower layer show evidence of large-scale destruction of habitations by fire.（A）
(E) Bronze ax heads of a type used at the time of the siege were found in the lower level of excavation.
这题的题目看得稀里糊涂，不太明白他要问的是什么？ | 
