IMPLICATION OF PROBLEM BASED LEARNING ON ENVIRONMENTAL POLLUTION SUBJECT TO IMPROVE STUDENTS’ CRITICAL THINKING SKILL AT CLASS X MIA 3 OF SMA NEGERI 3 SURAKARTA - Suwarno, Hesti Retno Puji Astuti, Baskoro Adi P, (P Biologi FKIP UNS Surakarta, 2015)