单项选择题 Experts say walking is one of the best ways for a person to ______ healthy.

单项选择题 Scientists generally agree that the Earth's climate will warm up over the next 50 to 100 years ______it has warmed in the 20,000 years since the Ice Age.

单项选择题 Physics is the present-day equivalent of ______ used to be called natural philosophy, from which most of present-day science arose.