I don’t think one can keep healthy _____ taking enough exercise.[ ]

I often get up ______ about 9 o’clock ______ Sundays. [ ]

This exercise is ________ difficult ________. [ ]

Is that boy Alan?—No, it ______ be him. He left for Beijing yesterday.

Have you ever seen ____ pen? I can’t find my pen anywhere. –Is it ____blue one? It is under the book.