URL addres

http://digilib.unimus.ac.id/files/disk1/105/jtptunimus-gdl-ekosetiyaw-5249-3-bab2.pdf
http://etd.eprints.ums.ac.id/2971/1/B100040465.pdf
http://digilib.unnes.ac.id/gsdl/collect/skripsi/archives/HASH3526/65eba7f5.dir/doc.pdf
http://etd.eprints.ums.ac.id/3840/1/A210050110.pdf