I believe it is element As, for arsenic.
Answer:
part a: How much of the isotope will you have left after 10 years?
part b: How much of the isotope will you have left after 20 years?
part c: How many half-lives will occur in 40 years?
The answer is D. KOH (potassium hydroxide)
Im not that’s sure because it’s very confusing let me find out wait