I met a friend of mine _____ in the street yesterday .[ ]

Is this the reason __________ at the meeting for his carelessness in his work?[ ]

Look! There is a big river. Let’s swim in it!—No, I think it’s _________ dangerous.

Which is ________ to learn, singing or dancing, baby?

Look at that girl! Is that Susan?—No , it _______ be her. She has gone back to her hometown.