单项选择题The underlined word upbeat in paragraph 2 probably means ______.
单项选择题The author believes that more houses were bought last year because______.
单项选择题The weekly unemployment data show that ______.
单项选择题The author discusses computing machines in the first paragraph primarily in order to ______.
单项选择题The author suggests that a mathematician asked to solve a problem in an ill-defined situation would first attempt to ______.