IMixing combined with skip voting
Remark
The given thing on the right is a positron. The mass for these subatomic particles is considered to be 0. It's atomic number is 1 which means it is a blood relative of a proton.
So essentially what happens is that X is one space to the left on the periodic table. But let's solve this a little bit more formally.
Solution

y stays the same at 147. It is z that changes.
65 = z + 1 Subtract 1 from both sides.
64 = z
So the chemical with 64 as its position on the periodic table is
Gadolinium and the answer is C
The density at 1,291 km from the surface of the planet is
[ (1,291 - planet's radius)/(planet's radius) ] ^ some power .
Answer:
Transition metals, alkali metals, alkaline earth metals Transition metals - Middle of the periodic chart, only average reactivity. alkali metals - As mentioned above, very reactive. Bad choice, going from lower reactivity to higher reactivity.
Hope this answer is right!