【单选题】
在下图所示电路中，电路______的输出不为高电平。
①
<imgwidth="100"height="45"src="http://wljy.whut.edu.cn:80/uploadfiles/word/szlj05.files/szlj056501.png">
②
<imgwidth="123"height="45"src="http://wljy.whut.edu.cn:80/uploadfiles/word/szlj05.files/szlj056504.png">
③
<imgwidth="112"height="48"src="http://wljy.whut.edu.cn:80/uploadfiles/word/szlj05.files/szlj056507.png">
④
<imgwidth="124"height="59"src="http://wljy.whut.edu.cn:80/uploadfiles/word/szlj05.files/szlj056510.png">