单选题
The newly installed alarm system, which is directly connected to the police, will go off if the correct security code is not entered ______ 60 seconds of touching the keypad.
【正确答案】
C
【答案解析】备选答案为副词和介词。首先应分析句子结构，判断横线处应填入介词还是副词。乍一看应填入副词修饰被动结构is not entered，实际上if从句是被动语态，______60 seconds of touching the keypad应作状语。因此横线处应填入介词，从而构成介宾短语作状语。