[求助]大全GMAT1-19,20

请问思路,看不太懂题目的意思耶!
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.答案C,我选B(虽然觉得蛮蠢的)
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.答案:A,我选B
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 ax heads of a type used at the time of the siege were found in the lower level of excavation.

寻找传说中一个城市围攻和毁灭地点的考古学家正在挖掘几个可能的地方，包括a large mound的middle layer和lower layer ，middle layer的底部包含pottery of type 3，type 3是城市毁灭后的东西，lower layer 没有type 3。

19归纳题，C说middle layer不能代表围攻的时期。因为middle layer里面的东西不是那个时期的。故为答案。

20削弱题：

A指出一种小动物在这个地方挖吭然后将东西带进去。说明middle layer的type 3可能是这种小动物带进去的。故削弱。