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which sentence has a verb that is in the active voice?

a. in 1854, the ideas behind electronic transmission of human speech had been developed by charles bourseul.

b. in 1955, engineers placed the first transatlantic telephone cable between newfoundland, canada, and scotland.

c. in 1915, a telephone link between new york city and san francisco was established by engineers.

d. in 1878, the world’s first commercial telephone company was opened by entrepreneurs in connecticut.