单选题 It was not until the accident happened ______.
A. that I realized my carelessness
B. when I realized my carelessness
C. as I realized my carelessness
D. when my carelessness has been realized

【正确答案】 A
【答案解析】此句是由“It is/was+被强调部分+that/who．．．”构成的强调句型，被强调部分为状语。