The molar specific heat capacity of a diatomic gas at constant pressure is \( C \). The molar specific heat capacity of a monatomic gas at constant volume is
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For specific heat relations, remember \( C_P - C_V = R \) and apply ratio-based methods when comparing different gases.
For a diatomic gas:
\[
C_P = C_V + R
\]
Since \( C_P = C \), we get:
\[
C_V = C - R
\]
For a monatomic gas:
\[
C_V' = \frac{3}{2} R
\]
Using \( C_P = \frac{7}{2} R \), we write:
\[
C_V' = \frac{3}{7} C
\]
Thus, the correct answer is \( \frac{3C}{7} \).