Fill in: in, on (x2), out, at, off, against, into, from, down
1) people who live the streets are risk of getting sick
2) turn the music or switch it completely - i'm studying
3) the rapid decline back rhinos means time is running for these wonderful creatures
4) the group is fighting scientists experimenting animals
5) illegal immigration is turning a problem that also affects those who arrive abroad legally.

1) People who live **on** the streets are **at** risk of getting sick.

Explanation: "On" is used to indicate location, and since the sentence is referring to the location where people live, we use "on" the streets. "At" is used to indicate a specific point or place, so we use it to indicate that the risk is present for those individuals.

2) Turn the music **off** or switch it completely - I'm studying.

Explanation: The phrase "turn off" means to stop or disable something, particularly in this case, the music. When studying, it is important to eliminate distractions, which is why we want to turn the music off.

3) The rapid decline **in** back rhinos means time is running **out** for these wonderful creatures.

Explanation: "In" is used to indicate a decrease or decline, so we use it to describe the decline in the population of rhinos. "Out" is used to indicate that a specific amount of time remains or that something is ending, so it is suitable in this context to imply that time is running out for the rhinos.

4) The group is fighting **against** scientists experimenting **on** animals.

Explanation: "Against" is used to indicate opposition or resistance, so we use it to describe the fight between the group and the scientists. "On" is used to indicate the subject or object of an action, so in this context, we use it to describe the scientists conducting experiments on animals.

5) Illegal immigration is turning **into** a problem that also affects those who arrive **from** abroad legally.

Explanation: "Into" is used to indicate a transformation or change, so we use it to describe how illegal immigration is becoming a problem. "From" is used to indicate the origin or source, so we use it to describe where those individuals who arrive legally are coming from.