Answer:
(A) fossil is the most useful.
It serves as an index fossil. The seashell is a great index fossil because it only appears in the sandstone layer in multiple locations in the same layer of time.
Explanation:
The relative molecular mass of the gas : 64 g/mol
<h3>Further explanation</h3>
Given
Helium rate = 4x an unknown gas
Required
The relative molecular mass of the gas
Solution
Graham's Law

r₁=4 x r₂
r₁ = Helium rate
r₂ = unknown gas rate
M₁= relative molecular mass of Helium = 4 g/mol
M₂ = relative molecular mass of the gas
Input the value :

Answer:
because no chemical change is happening
Explanation: