The flu is believed ________ by viruses that like to reproduce in the cell inside the human nose and throat.

Much attention should be paid _______ the wild animals as many of them are dying out.  

__________ with such great difficulty, Jack felt at a loss__________.

        with the size of the whole earth , the biggest ocean does not seem big at all .

The organizers announced that the first ______ at the top shall receive a big prize.