国民党之所以对陕甘宁解放区发动进攻，主要目的是（  ）

阅读《钱塘湖春行》回答下面的问题。

                                                                                                                                 钱塘湖春行

                                                                                                                                唐  白居易

                                                                                                            孤山寺北贾亭西，水面初平云脚低。

                                                                                                           几处早莺争暖树，谁家新燕啄春泥。

                                                                                                           乱花渐欲迷人眼，浅草才能没马蹄。

                                                                                                           最爱湖东行不足，绿杨阴里白沙堤。

如图，在矩形ABCD中，对角线AC，BD相交于点O，若∠AOD=2∠AOB，AB=4cm，则矩形ABCD的面积是____cm²．

直接写出得数。