Thinking that the students were having an evening class, the teacher left the classroom, _____.

Mike was usually so careful, ______this time he made a small mistake.

 A woman in her 60s         gave birth to two test tube babies three years ago, after her only daughter died of gas poisoning.

The student went through the paper, finding some mistakes that might         have been ignored．

Food safety is ________ important, so the government spares no efforts to prevent food pollution.