解析：that　句意：通常情况下，对于那些坚持希望的人来说，一切皆有可能。空处引导主语从句，从句中不缺少任何成分且意思完整，故使用that引导。

　　6．(2015·全国卷Ⅱ)After looking at the toy for some time, he turned around and found where his parents were missing.________________

　　解析：where→that或去掉where　found后是一个宾语从句，不缺少句子成分且句意完整，所以不能用where，应该用that引导，that也可省略。

　　[谨记规则]

　　(一)that引导的名词性从句

　　that引导名词性从句时本身无意义，只起连接作用，也不在句中作任何成分。它引导的名词性从句结构和意义都完整，往往用于陈述事实。引导主语从句、表语从句和同位语从句时，that不能省略。引导宾语从句时，在口语和非正式文体中，that可省略。但下列情况下，that不可省略：

　　1．that从句作介词的宾语；

　　I don't know exactly where he lives, except that it's somewhere out in the wilds.他具体住在哪里我不是很清楚，只知道是远离城市的边远地区。

　　2．that引导的从句位于句首时；

　　That the earth goes around the sun is known to us all.

　　众所周知，地球围着太阳转。

　　3．宾语从句前有插入语时；

　　We hope, on the contrary, that he will stay at home with us.恰恰相反，我们希望他和我们待在家里。

　　4．动词后有多个that引导的宾语从句时，只有第一个that可省略，从第二个从句开始that不可省略；

　　He told me (that) he had to leave and that he would be back soon.他告诉我他得离开并且很快就会回来。

　　5．it作形式宾语时，that引导的宾语从句中that往往不可省略。

　　He has made it clear that the meeting will not be postponed.他清楚地表明会议不会被推迟。

　　(二)that引导主语从句时可用it作形式主语的四大句式

1．It＋be＋形容词(necessary, right, likely, unlikely, wrong, important, certain, clear, obvious, strange, normal等)＋that从句