The idea has been discussed to "terreform" the planet Mars, to make its climate more like Earth's, by crashing comets into it. What effects would be helpful? (More than one answer possible)

a. It would knock the planet into a more favorable orbit, nearer the Sun. 

b. It would release water vapor (a greenhouse gas), which would warm the atmosphere.

c. It would increase the atmospheric pressure, so that water could exist on the surface in liquid form.

d. It would add oxygen to the atmosphere.

e. All of the above.