gwd1-8

It is theoretically possible that bacteria developed on Mars early in its history and that some were carried to Earth by a meteorite.  However, strains of bacteria from different planets would probably have substantial differences in protein structure that would persist over time, and no two bacterial strains on Earth are different enough to have arisen on different planets.  So, even if bacteria did arrive on Earth from Mars, they must have died out.

The argument is most vulnerable to which of the following criticisms?

A.      It fails to establish whether bacteria actually developed on Mars.

B.      It fails to establish how likely it is that Martian bacteria were transported to Earth.

C.      It fails to consider whether there were means other than meteorites by which Martian bacteria could have been carried to Earth.

D.      It fails to consider whether all bacteria now on Earth could have arisen from transported Martian bacteria.

E.       It fails to consider whether there could have been strains of bacteria that originated on Earth and later died out.

我选c

why the answer is D？who can tell me the reason

c是无关选项，因为即使不是meteorites带来的，Martian bacteria 也会遵循“两个星球的细菌在蛋白质结构上有本质不同”这个规律，不能weaken原文。

D是正确的，因为如果地球上的细菌全是Martian bacteria那么他们自然不会有本质的区别（本是同根生嘛），也就不能用这个标准判断他们是否土生土长于地球。

希望我说明白了