既是维持地表系统正常运行的动力，也是联系岩石圈、大气圈、水圈、生物圈的桥梁和纽带是（    ）

阅读下面一首诗，完成下面题目。
                                                                                                                               无 题
                                                                                                                               李商隐
                                                                                                      相见时难别亦难，东风无力百花残。
                                                                                                      春蚕到死丝方尽，蜡炬成灰泪始干。
                                                                                                      晓镜但愁云鬓改，夜吟应觉月光寒。
                                                                                                       蓬山此去无多路，青鸟殷勤为探看。

直接写得数。

缩写句子。