Table 4. Keyword frequency that appeared from 1920 to 1999

Ranking	Keyword(Noun)	Fre.	Ranking	Keyword(Noun)	Fre.	Ranking	Keyword(Noun)	Fre.
1	Urban	782	11	Our country	281	21	Life	163
2	Environmental	627	12	Reality	274	22	Social issue	152
3	Foreign country	610	13	Waste	263	23	Population	133
4	Regeneration	463	14	Development	237	24	Art	133
5	Environmental pollution	382	15	Problem	213	25	Next	129
6	Culture	367	16	Agriculture	210	26	Disposal	128
7	Green space	335	17	Metropolitan area	198	27	Government	124
8	Economy	290	18	Human	196	28	Factory	122
9	Garbages	285	19	Industry	185	29	Local	118
10	Rural	284	20	Countermeasure	172	30	Production	117