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The rapid growth in population in the early nineteenth century was once thought to be related to improvements in medicine.
→ 능동태: People once thought that the rapid growth in population in the early nineteenth century was related to improvements in medicine.

People might expect that these waves of immigration would exert an influence on the development of English in America.
→ 수동태: It might be expected that these waves of immigration would exert an influence on the development of English in America.