The elements are arranged by their atomic number but are also grouped by what type of element they are.
For example, metals, non metals, metalloids, and noble gasses will be grouped with elements that match them.
Hope this makes sense!
CaI₂ + Hg(NO₃)₂ --------->HgI₂ + Ca(NO3)2
2Al + 3Cl₂ --------->2AlCl3
Ag + HCl ------->AgCl + H2
C2H2 + 5O2 --------> 4CO2 + 2H2O
MgCl₂ --------->Mg + Cl2
Answer:
B They move because of the convection currents in the mantle.