Failure can make a positive contribution to your life you learn to use it.A. sinceB. unle

问题补充：

Failure can make a positive contribution to your life you learn to use it.

A. since

B. unless

C. until

D. once

答案：

A. since

B. unless

C. until

(答案→)D. once

解析：考查连词：A. since既然，B. unless除非，C. until直到，D. once 一旦，句意是：失败可以对你的人生起积极的作用，一旦你学会使用它。选D。