单项选择题
According to the passage, which of the following is a necessity（）
A．Shortened opera performances. B．Quickened paces of movies. C．Speeded-up work efficiency. D．Working on holidays.
单项选择题 The author states that birds left the tropics because （）
单项选择题 The author’s purpose in writing the passage is （）
单项选择题 The main topic of the passage is （）
