Some experts have said that children believe they can succeed they will never become to

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。