III. Change these sentences to the passive voice :
71. For centuries, poets, writers and musicians have mentioned ao dai in poems, novels and songs.
72. They are going to plant some flowers and plants in our school yard next week.
73. Students in secondary school shouldn’t drive motorbikes at school.
74. They had opened a new museum in this land before we came to live in Vietnam in 1999.
75. People said that Michael Jackson died of using too much cocaine.
76. Their boos made them work very hard until midnight.
77. This is the first time they have written to us about this.
78. How many lessons are you going to learn next month?
79. Someone had invented electric lights before I was born.
80. People know that Japan produces a wide range of cars.