I would say the last one. Yes, the coral reef in the south is being threaten but, it’s not the only one. Many other reefs are being threatened. Why? To find the problem to this reef you need to find the problems of other reefs and compare the data. Which ever the most problem cause you get rid of that first.
Density - Calculated from measurements of mass and volume
Atom - the basic particle of matter
Motion - calculated from measurements of distance and time
Energy - can change form and move matter
Can't see the last one but you know process of elimination soo
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