Pratama, A.T. 2018. Improving metacognitive skills using problem based learning (pbl) at natural science of primary school in deli serdang, indonesia. Biosfer: Jurnal Pendidikan Biologi. 11, 2 (Nov. 2018), 101–107. DOI:https://doi.org/10.21009/biosferjpb.v11n2.101-107.