http://repository.umy.ac.id/bitstream/handle/123456789/18088/Bab%20II.pdf?sequence=4&isAllowed=y
http://repository.ump.ac.id/1263/3/ANGGI%20FAHMI%20LUCKYAMA%20BAB%20II.pdf
http://eprints.unram.ac.id/8160/1/KTI%20BAIQ%20ZULHAENI%20AL.pdf
https://repository.unej.ac.id/xmlui/bitstream/handle/123456789/104147/Rhevy%20Asril%20Hudaiva%20-%20162310101070%20Sdh.pdf?sequence=1
