source:Source: https://repository.ung.ac.id/get/karyailmiah/8535/Rasuna-Rasid-Talib-Laporan-Akhir-Penelitian-Produk-Terapan-Pengembangan-Buku-Ajar-Berbasis-English-For-Specific-Purposes-ESP-Melalui-Pendekatan-Lesson-Study-Di-Fakultas-Ilmu-Sosial-Universitas-Negeri-Gorontalo.pdf
