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1 . Diana Northup has curiosity . She wants to know about life in this cave . That means the cave ____________________ ____________________ ____________________ ( must / visited ) . Caves are dangerous places , and some risk ____________________ ____________________ ____________________ ____________________ ( have to / taken ) . Northup is always careful .

2 . She knows the dangers . For example , Northup knows that the air in this cave is poisonous , so she uses a gas mask . The cave ____________________ ____________________ ____________________ ( mustn't / enter ) without a gas mask .

3 . Northup knows that the level of gas goes up and down . When it goes up , it is extremely dangerous . That's why a gas meter ____________________ ____________________ ____________________ ____________________ ( have to / use ) . With this , the gas level ____________________ ____________________ ____________________ ( can / checked ) while she is down there .