it's the last one because it's depended on the rock layers
Because the ring is hollow
Answer:

Explanation:
Given,
The density of Helium is 
We need to find the density in Dg/μm
We know that,
1 g = 10 dg
1 cm³ = 10¹² μm³
So,

So, the density of Helium is equal to
.
They use it because it's standerize and evidence vase
B because it’s got the least connections