It is probably within the reach of human technology to make the climate of Mars inhabitable. It might be several centuries before people could live there, even with breathing apparatuses, but some of the world's great temples and cathedrals took centuries to build. Research efforts now are justified if there is even a chance of making another planet inhabitable. Besides, the intellectual exercise of understanding how the Martian atmosphere might be changed could help in understanding atmospheric changes inadvertently triggered by human activity on Earth.

The main point of the argument is that
A. It is probably technologically possible for humankind to alter the climate of Mars.
B. It would take several centuries to make Mars even marginally inhabitable.
C. Making Mars inhabitable is an effort comparable to building a great temple or cathedral.
D. Research efforts aimed at discovering how to change the climate of Mars are justified.
E. Efforts to change the climate of Mars could facilitate understanding of the Earth's climate.

OA is D
But the question also reveals that A is correct too. Could I be wrong? I need an Expert contribution. Thanks.