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An edge-cut $F$ of a connected graph $G$ is called a‎ ‎restricted edge-cut if $G-F$ contains no isolated vertices‎. ‎The minimum cardinality of all restricted edge-cuts‎ ‎is called the restricted edge-connectivity $λ'(G)$ of $G$‎. ‎A graph $G$ is said to be $λ'$-optimal if $λ'(G)=\xi(G)$‎, ‎where‎ ‎$\xi(G)$ is the minimum edge-degree of $G$‎. ‎A graph is said to‎ ‎be super-$λ'$ if every minimum restricted edge-cut isolates‎ ‎an edge‎.   ‎In this paper‎, ‎first‎, ‎we provide a short proof of a previous theorem about‎ ‎the sufficient‎ ‎condition for $λ'$-optimality in triangle-free graphs‎, ‎which was given in‎ ‎[J‎. ‎Yuan ‎and‎ ‎A‎. ‎Liu‎, ‎Sufficient conditions for $λ_k$-optimality in triangle-free‎ ‎graphs‎, ‎Discrete Math‎., ‎310 (2010) 981--987]‎. ‎Second‎, ‎we generalize a known‎ ‎result about the sufficient‎ ‎condition for triangle-free graphs being super-$λ'$ which was given by‎ ‎Shang et al‎. ‎in [L‎. ‎Shang ‎and‎ ‎H. P‎. ‎Zhang‎, ‎Sufficient conditions for graphs to be $λ'$-optimal and super-$λ'$‎, Network}, 309 (2009) 3336--3345]‎.