The answer here is D, and it's kind of obvious. But you probably already knew the answer and are just doing this for piece of mind.
Explanation: Just the fact that we don't know whether or not there even is extraterrestrial life rules out every other answer, without even needing to go in depth.
(Okay, I read your question completely wrong, it's asking for which one of these has been solved isn't it?)
This is tough because all of these other than D have been figured out. Go with the methane atmosphere then.
Recombinant DNA technology
Most of the water reaching the collecting ducts is not reabsorbed
Answer: Diffusion
Explanation:
Diffusion is the process or a spontaneous movement of particles from an area of higher concentration to an area of lower concentration.
Examples are;
1. perfume
2. A drop of ink in a cup of water.
From these two examples when it is dropped, especially ink, it diffused inside the entire space of the cup and fill all the space by penetrating the entire molecule of the water available.
