What Is the Most Significant Drawback of Possibly Sending Humans

What is the most significant drawback of possibly sending humans to Mars?

A) the fact that there would be no possible source of water on the surface
B) the high probability of contamination of the Martian surface by microbes associated with human beings
C) the high probability of Martian microbes contamination of contaminating the crew
D) the fact that astronauts could not breathe in the Martian atmosphere