The door ____ lock .
I have been told to donate blood by the health expert __________ I need some in the medical treatment one day.
Nelson Mandela _________ his life to fighting for a free society________ all people live together in harmony.
The number of various educational programs is increasing, _______ important roles have been realized by more and more people.
It is dangerous to take ________ from rain under a tree especially during a summer rainstorm.
--- What do you think of the grammar book?--- Oh, it really is ________ useful reference book, which is worth reading ________ second time.