Read the following passage and answer the question below.
"The monarch butterfly, with its vibrant orange and black wings, is a marvel of nature. Each fall, millions of these delicate insects embark on a monumental journey, migrating up to 3,000 miles from the northern United States and Canada to their overwintering grounds in central Mexico. This multigenerational migration is a unique and complex phenomenon. The generation that flies south is not the same one that returns north the following spring. It takes several generations to complete the round trip. Unfortunately, the monarch population has seen a steep decline in recent years due to habitat loss, climate change, and the reduction of milkweed, the sole food source for monarch caterpillars."
What is the main idea of the passage?
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A
Monarch butterflies have vibrant orange and black wings.
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B
The monarch butterfly's multigenerational migration is a remarkable natural event that is currently under threat.
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C
Milkweed is the only plant that monarch caterpillars eat.
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D
Monarch butterflies travel to Mexico because of climate change.