反证法证明的关键是在正确的假设下得出矛盾,这个矛盾可以是(    )

①与已知矛盾；②与假设矛盾；③与定义、定理、公理、法则矛盾；④与事实矛盾

 过氧化钙晶体（CaO₂•8H₂O）较稳定，呈白色，微溶于水，广泛应用于环境杀菌、消毒．以贝壳为原料制备CaO₂流程如下：

根据汉语提示完成下列句子，每空一词。
____ Do you often do                       (户外活动) after school
____ There is                          (一些有趣的事情) in my class.
____ Thank you for their                         .（辛勤工作）
____ Please                      ( 穿上 ) your coat. It's cold outside .
____ Lucy is a good girl and she always returns the books to the library                  (准时)。