(2013·江苏南通市二模)We ________ have hurried all the way to the airport — the flight was called off because of the foggy weather.

(2013·新课标Ⅰ，29)The door________open，no matter how hard she pushed.

You'd better take a heavy coat. Sometimes it ______ be very cold.

Teachers should ___them to continue studying when some students want to give it up. 

If you ___ stick to the unrealistic approach, the worse result will be waiting for you not far ahead.