Online paper-checking requires that more attention _______.

—He looks very happy. He ____have passed the exam, —I guess so. It’s not difficult after all.

The boy suggested that Tom ______ the problem another way, and Tom’s expression suggested that he_________. 

 To be on the safe side, we should fill up the tank now, because we ______ run out of gas on the way.

---- Did you visit the Big Ben in London?---- No, we ______ it, but we spent too much time shopping.