She ______ know the answer, but I’m not sure.

Brain, like muscles, also needs some kind of exercise and nutrition. The more you use your brain in the right direction, the more you ________ enhance your ability.

— I think the man over there must be Bob.— It _______ be him. He has gone to England.

That notebook _____  John’s. It has his name on it. 

— I phoned you yesterday morning. A girl answered, but I didn’t recognize the voice.— Oh, it ____ my younger sister. She was in my room at that moment.