Radioactive material obeys 1st order decay kinetics,
For 1st order reaction, we have
k =

where, k = rate constant of reaction
Given: Initial conc. 100, Final conc. = 6.25, t = 18.9 hours
∴ k =

= 0.1467 hours^(-1)
Now, for 1st order reactions: half life =

= 4.723 hours.
Hi! My answer would be D, USA.
Explanation:
We are provided with 5 pieces of an address. They are as followed.
12 Riverdale Lane, Apartment A, San Jose, California, and USA.
Imagine each of the five parts of Beck's address represents Earth, the Milky Way, the moon, the solar system, or the universe, based on their sizes. Which of the following parts of Beck's address would represent the solar system?
If we have to plug in Earth, the Milky Way, the moon, the solar system, and the universe, (all based on size,) we would first start with what "Apartment A" would be in this scenario. Apartment A is the smallest piece we have to work with, and so is the moon. This leads me to believe that the Moon should be plugged into Apartment A. Now, 12 Riverdale Lane is most likely going to be the Earth, since it is the second smallest one on our list. San Jose would turn into the Sun, California represents the Milky Way, and USA represents the solar system.
I sincerely hope I helped!
Have a wonderful day :)
~AmbitiousAndProud
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