An eclipse is when two celestial bodies lines up in space and one of the objects gets partially or totally blocked. This could happen when the sun blocks the moon, or when the moon blocks the sun. These are just some of the examples there are many others out there, so the answer would be Eclipse!
Hope this helps and stay safe!
The half life of Strontium-90 is 28.8 years
The answer you are looking four is 4/2 or 4CO2/2C2H6