Answer
b.
Work Step by Step
Mars is only about half the size of Earth, so it doesn't have much of a gravitational pull to be able to keep gases, including oxygen, from escaping into space; therefore, any organisms that may have lived or are living on Mars would be anaerobic.
Also, Mars has no ozone layer, so if it houses any organisms, these species would most likely live underground to escape the sun's UV radiation. The discovery that Mars may contain water could support the idea that Mars may have supported or continues to support life in some form.
