Why Is the Possible Contamination of the Earth by Martian

Why is the possible contamination of the Earth by Martian microbes in a future sample return mission not considered to be a significant problem?

A) Martian microbes will certainly be destroyed by terrestrial microbes
B) any Martian microbes will be destroyed by high-energy particles and radiation on their trip back to Earth
C) Martian microbes will be rapidly destroyed by oxygen in the Earth's atmosphere
D) Martian microbes are likely to be quite different than terrestrial microbes and, hence, will probably not be able to affect life on Earth