题目

Experience the newly opened Grand Canyon (大峡谷) West Skywalk in Colorado. Departing(离开) from Grand Canyon’s South Rim（外边缘） by Airplane to Grand Canyon’s West Rim, you will land and take a ground tour to the Skywalk! Walk on air for 70 feet over the edge（边缘） of Grand Canyon West. This Skywalk has been open since March 28, 2007. Daily visitorship to the Skywalk has been over 4,000 people. Please be patient to enjoy your moment on the Skywalk. After you have experienced the one and only Grand Canyon Skywalk Glass Bridge, you will return to the Grand Canyon Wes如下图所示，一根轻弹簧竖直直立在水平地面上，下端固定，在弹簧的正上方有一个物块，物块从高处自由下落到弹簧上端O，将弹簧压缩，弹簧被压缩了x0时，物块的速度变为零.从物块与弹簧接触开始，物块的加速度的大小随下降的位移x变化的图象可能是（    ）