单选题
It’s disturbing to note how many of the crimes we do know about were detected ___not by systematic inspections or other security procedures.
【正确答案】
A
【答案解析】by accident“偶然”；on schedule“按时”；in general“通常，大体上”；at intervals“不时”。根据句子结构，所填词与by systematic inspections or other security procedures是对称的，by accident 符合要求。