It was already one in the morning I saw the dull yellow eye of the creature open.
A. when
B. that
C. which
D. since
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(答案→)A. when
B. that
C. which
D. since
解析：去掉“It was”和空格，我们发现不能成为句子，故推断不是强调句，排除B选项。“one in the morning”告诉我们指的是“时间”，用关系副词when引导定语从句。句意：已经是凌晨一点钟我才看到那个家伙黄色的眼睛睁开了。since只能引导状语从句。