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.

(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.

为什么选A啊？我觉得最不可能的就是A了，请指教，谢谢！

A说这种动物打洞，然后如果洞坍塌的话就会有东西掉进去。。。也就是说，可能那些在中间层找到的证据是由上面一层掉下来的.