工人师傅在用方砖铺地时，常常打两个木桩，然后沿着拉紧的线铺砖，这样地砖就铺得整齐，这个事实说明的原理是______．

— You look upset. What's the matter? 

— I had my proposal _____ again. [ ]

A. turned over

B. turned on

C. turned off

D. turned down