Consider the following data:

| Decade | Birthrate | Death rate |

| 1901-1911 | 49.2 | 42.6 |

| 1911-1921 | 48.6 | 47.2 |

| 1921-1931 | 46.4 | 36.3 |

| 1931-1941 | 45.2 | 31.2 |

For which period was the natural growth rate (birthrate minus death rate) maximum?