Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
The atmosphere is a big part of the water cycle. What do you think would happen to Earth’s water without it?.


Question: What is the upper-most atmosphere known as?
thermosphere
Ionosphere or Exosphere. (Depending on the context, either one could be correct.)