Washing hands frequently is one of the effective ways to avoid ________ by diseases.
Experiments of this kind ________ in both the U.S. and Europe long before the Second World War.
-- Shall we take a taxi there since time is limited? -- ________. It should be hard to get one during the rush hour.
Traveling without ordering rooms in advance can eat up precious vacation time ________luck is smiling on you.
-- It’s more convenient to take a subway than drive a private car. -- ________. What’s more, it’s also a good way to support the low-carbon lifestyle.
It is known to all that the US is about the same size as China, whereas its population is five times________.