题目选项分析

Travelers from Earth to Mars would have to endure low levels of gravity for long periods of time, avoid large doses of radiation, contend with the chemically reactive Martian soil, and perhaps even ward off contamination by Martian life-forms.

Travelers from Earth to Mars, having to endure low levels of gravity for long periods of time, avoid large doses of radiation, plus contend with the chemically reactive Martian soil, and perhaps even ward off contamination by Martian life-forms.