春秋战国时期，我国农业出现了一次大变革，农业的深耕细作，使生产力得到了大发展。这些变化的主要原因是（    ）

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                                                                                                                                          新晴
                                                                                                                                          刘攽
                                                                                                              青苔满地初晴后，绿树无人昼梦余。
                                                                                                              唯有南风旧相识，偷开门户又翻书。

— I can't decide at present. ________.

有4个小朋友，每个小朋友都有2本书，一共有多少本书？乘法算式____。