1 . They say the noise caused by square dancing is ________ (……的主要原因) they are against it. (根据汉语提示完成句子)

2 . Scientist have searched for a long time for a reason ________ so many bees are disappearing. (用适当的词填空)

3 . Sadly, ____________________ the number of monarch butterflies is falling.   
不幸的是，人类活动是黑脉金斑蝶数量下降的主要原因。

4 . Human activity is ________(是……的主要原因) the number of monarch butterflies is falling. (根据汉语提示完成句子)