| A | B |
|---|
| challenge | something needing a great mental or physical effort in order to be done successfully |
| collapse | fall down suddenly, be unable to continue in operation |
| contagious | spread quickly to other people |
| disorder | an illness that causes a part of the body to stop functioning correctly |
| figure out | think about something until you understand it |
| magnetic | behaving like a magnet |
| migration | movement from one place to another |
| occur | happen |
| researcher | someone who does a detailed study of a subject in order to discover information |
| theory | something suggested as a reasonable explanation for facts |
