阅读下面的诗歌。
                                                                                              滁州西涧
                                                                                                韦应物
                                                                                        独怜幽草涧边生，
                                                                                       上有黄鹂深树鸣。
                                                                                       春潮带雨晚来急，
                                                                                        野渡无人舟自横。

下列函数中，最小正周期为$\pi$的偶函数为（    ）

汶川地震后，防化部队在灾区喷洒大量消毒液，从预防传染病的角度看，这是（    ）

下面哪项最能体现英国发动鸦片战争的本质目的(    )