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 The professor can hardly find sufficient grounds ______ his argument in favor of the new theory.