GWD6-Q40

[bmwmthree]

GWD6-Q40:

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.

I don't understand what the passage above say
and why answer is D?
Please help me out!!!

[Unbeatable]

其實很明顯，因為文章在談論的是地球的某些bacteria是否有可能是來自火星的! 而文章的結論是不可能，因為地球上雖然不同的細菌體內的蛋白質夠成不同，但是卻不足以構成明顯的差異，進而得知來自火星的可能性級低，即使有，也是死亡。