The most interesting thing to the small boy was he got close to the door, it would open of itself.
A. that as
B. that that
C. what that
D. what when
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(答案→)A. that as
B. that that
C. what that
D. what when
解析：句意：对于小男孩最有趣的事情是当他走近门的时候，门自动开了。这里that引导的是表语从句，that在从句中不作成分，只起引导的作用as引导的是时间状语从句。根据句意故选A。