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.

为什么是A, 题干也没读太懂， 百思不得其解。