Underline the correct modal verbs: 1) scientists don't have to/must consider the dangers of cloning. 2) people mustn't/don't have to be experts to operate a dvd player. 3) you must/mustn't switch off your computer when it is not in use. 4) she has to/mustn't be 20 years old now. 5) you have to/mustn't be careful with electricity. 6) people don't have to/mustn't take computer lessons to use a computer. 7) john mustn't/has to study hard if he wants to become a robotics engineer.