问题补充:
Some experts have said that children believe they can succeed, they will never become totally confident of themselves.
A. if
B. since
C. once
D. unless
答案:
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。