如图所示，入射光线与平面镜成α角，要使反射光线与入射光线之间的夹角增大20°（入射光线不动），则平面镜应（  ）

拼拼读读。

tong xué zhī jiān yào hù xiāng guān xīn，hù xiāng bāng zhù，bāng zhù bié rén shì zì jǐ zuì dà de kuài lè.

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____  ____ our farm!

阅读下面两段文字，按要求回答问题。
                                                                                             乡 愁
                                                                                           小时候
                                                                                           乡愁是一枚小小的邮票
                                                                                           我在这头
                                                                                           母亲在那头
                                                                                           长大后
                                                                                          乡愁是一张窄窄的船票
                                                                                          我在这头
                                                                                          新娘在那头
                                                                                          后来啊
                                                                                          乡愁是一方矮矮的坟墓
                                                                                          我在外头
                                                                                          母亲在里头
                                                                                          而现在
                                                                                         乡愁是一湾浅浅的海峡
                                                                                         我在这头
                                                                                        大陆在那头

无土栽培是指(   )