问一道补充材料里面的逻辑：）

mialanhero

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. 19. Which of the following hypotheses is best supported by the evidence above?
(A) The lower layer contains the remains of the city where the siege took place.
(B) The legend confuses stories from two different historical periods.
(C) The middle layer does not represent the period of the siege.
(D) The siege lasted for a long time before the city was destroyed.
(E) The pottery of type 3 was imported to the city by traders. 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.
(E) Bronze axheads of a type used at the time of the siege were found in the lower level of excavation. 答案分别是c和a。第一题的e选项为什么不可以呢？还有哪位nn能给俺讲讲其中的逻辑关系啊？