GRE Text Completion: JiJing 1373-GRE填空机经1373题 - HT1PDDIHVFY7F09NA

Considering how difficult it is to prove or disprove the existence of life on Mars, which is practically our neighbor, it is quite ____________ challenge to do the same for any Earth-like planets that might exist outside our solar system. A. a formidable B. an impossible C. a hopeless D. a daunting E. an urgent F. an immediate