⑴We can't swim in the lake?____                                        A.

⑵There is a toilet here.____                                                B.

⑶We should keep quiet in the library.____                         C.

⑷We can't walk on the grass.____                                       D.  

⑸We can eat here.____                                                        E.

a．现将AB突然竖直向上平移（平板与磁铁之间始终接触），并使之停在A″B″处，结果发现两个条形磁铁吸在了一起。

b．如果将AB从原来位置突然竖直向下平移，并使之停在位置A′B′处，结果发现两条形磁铁也吸在了一起，请回答下列问题：

每年的6、7、8三个月是北半球的(    )。