_____ his knowledge of the mountainous country, John Smith was appointed as our guide.
Everyone knows that________ is dangerous to play with fire, but________ is difficult is to prevent children from playing with fire.
One can ______ BBC _____ the air early with his Short-wave radio.
— How many teachers are party members in your school?— Party members_________ one fifth of the number of the teachers in our school.
Jim was born on a farm, _____, he knows little about farming.
______ is surprising to us is that Tom, for ______ English was once boring, can speak English fluently as though he were a native speaker.