On Saturdays, students are very proud to be         on the street to guide passers-by to obey traffic rules.

. Today, housework has been made much easier by electrical _____.

(2014·湖南师大附中一模)The first successful space walk in 2013 is an important historical________in Chinese history.

 The doctors must get there in half an hour. There should be no____ in sending the blood to the dying girl .

Dogs have a very good ______ of smell and are often used to search for survivors in an earthquake.