EWs of [CIII] (solid) and CIV (dashed) as a function of age for single- (left) and binary- (right) stellar population under the assumption of continuous star-formation. The age is defined since the beginning of the current star-formation. They are normalized at 1 Myr. The decrease of EW of [CIII] (CIV) is estimated by counting the number of photons with an energy of >24.4 eV (47.9 eV) divided by the UV (stellar+nebular) continuum around the [CIII] (CIV) emission at a given age. The blue, green, yellow, and red curves denote the age dependencies for a metallicity of Z = 0.1, 0.2, 0.5, and 1.0 Z_⊙, respectively.