Invariant mass distributionsof $\pi^+\pi^-$ pairs in the four multiplicit-$k_{\rm T}$ ranges used in the study. K$^0_s$ used in this analysis were identif${\rm i}$ed by the cut 0.490 GeV/$c^2$ $< m < $ 0.504 GeV/$c^2$. Also shown is a Gaussian+linear f${\rm i}$t to the data points.