Answer:
The theoretical density for Niobium is
.
Explanation:
Formula used :

where,
= density of the unit cell
Z = number of atom in unit cell
M = atomic mass
= Avogadro's number
a = edge length of unit cell
We have :
Z = 2 (BCC)
M = 92.91 g/mol ( Niobium)
Atomic radius for niobium = r = 0.143 nm
Edge length of the unit cell = a
r = 0.866 a (BCC unit cell)


On substituting all the given values , we will get the value of 'a'.


The theoretical density for Niobium is
.
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I believe it is D, System Controller.
It is responsible for opening things in the body like a valve. I also did my research and had a little help on this.