Monarch butterflies travel long distances to stay warm. They fly up to 3,000 miles to the same winter roosts, sometimes to the exact same trees. However, their life span is only a few months ____________. Their great-great-grandchildren return south the following fall.

• A. baby swallows and their parents learn each other's voices
• B. most of the bird species migrate south for the winter
• C. other butterflies also fly thousands of miles in search of warmth
• D. so individual monarchs only make the round-trip once