Old people need special care in winter, as they are very __________ to sudden change of the weather.
We should think of ways to stop the waste from the factory _____ our living environment.
As far as I know, this man ____ risk anything than lose the ring for his wife.
She locked her room _____ a thief would break into it.
She said she would spend her holiday ___ Beijing.
Whenever I go to Beijing, ____ is quite frequent, I will go to visit the Great Wall.