—Who______ it be that is knocking at the door? —It ______ be Father, but I’m not sure.
____ the floor, I found his pen lying on the ground.
To master a foreign language, ______.
With a lot of difficulties _______, they went to the seashore and had a good rest.
It is difficult to imagine his ______the decision without any consideration.
__________ by many tall trees, the lake looks more beautiful.