题目

Greenberg was a lucky guy to enter Columbia University on full scholarship． But just before junior year，Greenberg's______ changed． One day his vision ______ up and later he was diagnosed （诊断）with glaucoma （青光眼）． Then doctors operated on Greenberg's eyes．______，the surgery didn't work． Greenberg was going blind． He was so______ that he refused to see anyone．As Greenberg' s best friend，Arthur persuaded Greenberg to go back to Columbia and______ to be his reader． Arthur read ______ to him every day， taking time out from his own study， and Greenberg_____如图，在边长为1的等边三角形ABC中，点D是AC的中点，点P是BC边的中垂线MN上任一点，则PC+PD的最小值为 ．