Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 38 to 42.

Dolphins are one of the most intelligent species on the planet, which makes them a very interesting animal to scientists. In their natural habitats, dolphins use various vocalization techniques. They whistle and squeak to recognize members of their pod, identify and protect their young, and call out warnings of danger. They also make clicking sounds used for echolocation to find food and obstacles in dark and murky waters.

Amazingly, the whistling sound that the bottlenose dolphin makes has been found to have a similar pattern to human language. They always make conversational sounds when they greet each other. If you listen to dolphins' squeaks and squeals, it will sound like they are having a conversation.

Dolphins usually use both sound and body language to communicate with each other. It is through gesture and body language, however, that most of their communication with humans comes. Dolphins can be trained to perform complicated tricks. This suggests they have a high level of intelligence and communication capacity. If they work for a long time with a trainer, they are able to recognize and understand human commands.

A lot of dolphin communication has been studied using dolphins in captive environments. These studies have been criticized because some marine biologists believe that dolphins living in aquariums or research centers cannot be considered "normal." Even so, most believe that studying dolphin communication in captivity is useful for beginning to understand the complexity of dolphin communication. After all, dolphins are one of the most intelligent animals. Their ability to communicate is impressive and worthy of study.

In line 2, the word vocalization is closest in meaning to____________