The chairman thought _____ necessary to invite Professor Smith to speak at the meeting.

—Which of those electronic dictionaries do you like most?—     .They are expensive and of little use.

It is believed that intelligence is to _____ extent _____ we are born with.

I am blessed with a great class, ________everyone is hardworking, kind and friendly to each other.

Charles Darwin, a young man just out of university, knew perfect well that he had to depend on _____ when he set out on a voyage of discovery around the world.