Some experts have said that children believe they can succeed, they will never bee totally confident of themselves.
A. if
B. since
C. once
D. unless
网友回答
A. if
B. since
C. once
(答案→)D. unless
解析：D 考察状语从句。根据语境可知，unless引导条件状语从句，if如果，since自从，once曾经，一旦；句义：一些专家说，除非孩子们相信他们能够成功，否则他们永远不会对自己完全自信。故D正确。