Part ThreeDirections: In this section, you will hear four short conversations and two long conversations. At the end of each conversation, you will hear four questions. Both the conversation and the questions will be spoken only once. After you hear a question, you must choose the best answer from the four choices marked A, B, C and
D:Long conversation 115. What is the main idea of the conversation?
选项：

A:Dolphins are as clever as human beings.;
B:Dolphins can be taught to communicate with each other.;
C:Male and female dolphins have different responses to signals.;
D:Dolphins have different responses to different signals.