问题
单项选择题
The author refers to the ocean bottom as a "frontier" in Paragraph 1 because it ______.
A．is not a popular area for scientific research
B．contains a wide variety of life forms
C．attracts courageous explorers
D．is an unknown territory
答案
参考答案：D
解析：[分析] 推断题型。 参看第一段第二行...vast frontier that even today is largely unexplored and uncharted．