Questions 18 to 20 are based on the following passage. At the end of the passage, you will
be given 15 seconds to answer the questions. Now, listen to the passage. 18. The best statement of the main idea of this passage is that____A．human brains differ considerablyB．the brain a person is born with is important in determining his intelligenceC．environment is crucial in determining a person’s intelligenceD．persons having identical brains will have roughly the same intelligence