Pratama, Anggi Tyas. “Improving Metacognitive Skills Using Problem Based Learning (pbl) at Natural Science of Primary School in Deli Serdang, Indonesia”. Biosfer: Jurnal Pendidikan Biologi, vol. 11, no. 2, Nov. 2018, pp. 101-7, doi:10.21009/biosferjpb.v11n2.101-107.