On the Size and the Approximability of Minimum Temporally Connected Subgraphs

They provide a construction of a dense temporal graph where all edges are critical for TC — thereby showing that there exist strict/non-strict temporal graph where every temporal spanner has size $\Theta(n^2)$.
They study the approximability of two computational problems: Single-Source Minimum Temporal Connectivity (SS-MTC) and All-Pairs Minimum Temporal Connectivity (AP-MTC), which are natural temporal extensions of the static problem Minimum Spanning Tree (MST)