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.
Answer:
testing to see if it has cells
Explanation:
Super heated liquid molten lava
i think that before the Krebs cycle begins, pyruvic acid, which has three carbon atoms, is split apart and combined with an enzyme known as CoA.
Answer:
This is the digestive system.
Explanation:
We can tell that this is the digestive system because we can see #1: The stomach, #2: The small intestine, #3: The large intestine, and #4: The liver. Hope this helped!
