An index fossil is a fossil that is useful for dating and correlating the strata in which it is found.
Or it is a fossil that helps you find the date where it has been found. I rephrased that.
Its an element Im pretty sure
Answer:

Explanation:
Your nuclear equation is

The main point to remember in balancing nuclear equations is that
- the sum of the superscripts and must be the same on each side of the equation.
- the sum of the subscripts must be the same on each side of the equation.
Then
85 = 0 + y, so y = 85 - 0 = 0
35 = -1 + x, so x = 35 + 1 = 36
The nucleus with atomic number 36 and atomic mass 85 is krypton-85.
The nuclear equation becomes

Answer:
It is a material by chemically bonding two or more chemical elements
I think that work is being done on the books because they are being moved to their proper location and they will be sorted properly rather than lying on a table. Without lifting or carrying, you could sort the books by their genre or title name on the bookshelf so it will be sorted much more efficiently.
I’m not sure if this is the answer you are looking for but I hope it helps :)