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Q.
Carnot engine cannot give $100\%$ efficiency, because we cannot:
ManipalManipal 2004Thermodynamics
Solution:
The efficiency of Carnot engine is
$\eta=\frac{T_{1}-T_{2}}{T_{1}}=1-\frac{T_{2}}{T_{1}} \ldots$(1)
The value of efficiency of Carnot engine can be $1$ or $100 \%$, if the sink temperature $\left(T_{2}\right)$ must be $0\, K$. As practically absolute zero temperature can not be achieved hence, Carnot engine can not give $100 \%$ efficiency.