【单选题】
下列公式是前束范式的是( )
①
<imgwidth="158"height="20"src="../uploadfiles/word/lsss001.files/image019.png">
②
<imgwidth="186"height="20"src="../uploadfiles/word/lsss001.files/image020.png">
③
<imgwidth="143"height="20"src="../uploadfiles/word/lsss001.files/image021.png">
④
<imgwidth="167"height="20"src="../uploadfiles/word/lsss001.files/image022.png">