-- You know who came to our school yesterday?-- Professor Smith? We once met at a conference.-- ______. He came and gave a wonderful speech.
I like ____ in the autumn when the weather is clear and bright.
It was at four o'clock in the afternoon ____ he and his grandpa reached the museum in Guanghan.
I consider____ my duty to help you.
Is_possible to fly to the moon in a spaceship?
Is_necessary to tell his father everything?