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? ['It fails to consider whether all bacteria now on Earth could have arisen from transported Martian bacteria.', 'It fails to consider whether there could have been strains of bacteria that originated on Earth and later died out.', 'It fails to consider whether there were means other than meteorites by which Martian bacteria could have been carried to Earth.', 'It fails to establish whether bacteria actually developed on Mars.']