单项选择题 The 1986 Challenger space-shuttle （）was caused by unusually low temperatures immediately before the launch.

单项选择题 Others viewed the findings with （）noting that a cause-and-effect relationship between passive smoking and cancer remains to be shown.

单项选择题 In those days, executives expected to spend most of their lives in the same firm and, unless they were dismissed for （）to retire at the age of 65.