Consider the following data :

Year	Birthrate	Death rate
1911–1921	48.1	35.5
1921–1931	46.4	36.3
1931–1941	45.2	31.2
1941–1951	39.9	27.4
1951–1961	41.7	22.8
1961–1971	41.1	18.9
1971–1981	37.1	14.8

For which period was the natural growth rate maximum?