Directions: Complete the following passage by using the words in the box. Each word can only be used once. Note that there is one word more than you need.

    WhyHumpback Whales(座头鲸) Protect Other Species from Killer Whales

    Robert Pitman, a marine ecologist, describes an encounter he witnessed in Antarctica in 2009. A group of killer whales were attacking a Weddell seal. The seal swam  toward a pair of humpbacks that had inserted themselves into the action. One of the humpbacks rolled over on its back, and the seal was  onto its chest, between the whale's massiveflippers(鳍). "That incident  me," he says. "Those humpbacks were doing something we couldn't explain."

    Pitman started asking other researchers and whale watchers to send him similar . Soon he was reading through observations of 115 encounters between humpbacks and killer whales, recorded over 62 years. "There are some pretty astonishing videos of humpbacks  killer whales," he says.

    In a 2016 article inMarine Mammal Science, a famous scientific journal, Pitman and his co-authors describe this behaviour and confirm that such acts of do-gooding are widespread. But knowing that something is happening and understanding why it's happening are two different things. Pitman and his co-authors openly reflected on the meaning of these encounters. "Why," they wrote, "would humpbacks  interfere with attacking killer whales, spending time and energy on a potentially  activity, especially when the killer whales… were attacking other species of prey?"

    Interestingly, humpbacks don't just hit on killer-whale attacks. They race toward them like firefighters into burning buildings. And like those rescue workers, humpbacks don't know who is in danger until they get there. That's because the sound that  them to an attack isn't the sad voice of the victim. It's the excited calls of the killer whales. Pitman believes humpbacks have one simple instruction: "When you hear killer whales attacking, go break it up."

    I wonder what humpback whales care deeply enough about to actively swim into battle with killer whales. When I ask Pitman, he tells me that, it still comes down to selfishly  their own kind. He believes that their occasional rescues of humpbackcalves(后代) create a strong enough  for them to rush in to help, even if it means they end up saving sunfish, sea lions, dolphins every now and then.