Polymer (b) is an ethylene-based polymer, that is, a polymer made up mainly of ethylene, having a density of 0.91-0.97 g/cm3 and a melt flow rate at 190??? C. and a load of 21.18 N of 5-30 g/10 min, and preferably it is an ethylene homopolymer or a copolymer of ethylene and ??-olefin having a carbon number of 3 or more (for example, propylene, 1-butene, 1-pentene, 1-hexene or 1-octene) having a de