However, they dont envision obtaining soil samples from Mars until_____to try to ascertain

However, they dont envision obtaining soil samples from Mars until_____to try to ascertain whether there is water on the most Earth-like planet in our solar system and, possibly, life.

A．2001

B．2007

C．2011

D．2017

请帮忙给出正确答案和分析，谢谢！