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Question 77·Medium·Central Ideas and Details

Each autumn, millions of monarch butterflies begin a trek from the northern United States and Canada to the mountain forests of central Mexico. Because a single monarch lives only a few weeks, no individual completes the entire journey of thousands of miles. Scientists now think the feat is possible because temperature-sensitive "chemical clocks" are inherited by each successive generation. As daylight shortens, young monarchs produce lower levels of a hormone that would normally trigger reproduction. Instead of mating, these butterflies conserve energy for flight, nudging the population ever farther south toward wintering grounds with a cool, stable climate. In this way, the migration is less the work of a lone traveler and more the result of biological timers resetting with every new brood.

Which question does the text most directly attempt to answer?