78. Since the volcano last erupted, many houses have built on its slopes.
A B C D
⇒ have been
79. Since their usefulness, pesticides could pose potential risks to food safety.
A B C D
⇒ Due to
80. If people didn’t use cars, there will be much less pollution.
A B C D
⇒ would
Giải thích
78. have been câu này là THTHT
79. Since không đi với cụm danh từ
80. Câu ĐK loại 2 không dùng will