-You look upset. -Yes, this is the first time I________ form home.
No matter ______ the CRH accident in Wenzhou, it had and will have much warning significance.
It has been announced that all the students ____ put on masks before going to school in case they are infected with SARS virus.
—Have you been to the Great Wall?—Perhaps not in my memory. ________, it might have been during my early childhood.
The question that puzzled them is ____ they can get rid of the air pollution in the area.
Teachers have to constantly update their knowledge in order to maintain their professional _____.