The man kept silent in the room unless _______.
Without the friction (摩擦力) between feet and the ground, people would ______ be able to walk.
Don’t take his remarks too seriously. He was so sad that I don’t think he really _________ what he was saying.
--- Could you take a message for Mr Brown? --- Certainly. I ________ him about something else in any case. So it _______ any bother.
His aunt’s letters ________ memories of those beautiful days when they used to live together in his hometown.
The thing that _______ is not whether you fail or not, but whether you try or not.