应该是cross the desert通过沙漠.你可能区分的是介词across和through的用法.across 是"在平面上横过"(表示平面的) .如横过马路、操场、沙漠、广场等 through是"在空间穿过","穿透" (表示在立体的).如穿过森林、峡谷...two friends were walking ___ the desert是across，为什么答案里时through答案应该是错误的，一般都说go through the forests / valley... go across the desert