问题
选择题
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
答案：D
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。
