【单选题】
下列各函数等式中无冒险现象的函数式有______。
①
<imgwidth="104"height="29"src="http://wljy.whut.edu.cn:80/uploadfiles/word/szlj05.files/szlj055399.png">
②
<imgwidth="130"height="25"src="http://wljy.whut.edu.cn:80/uploadfiles/word/szlj05.files/szlj055425.png">
③
<imgwidth="165"height="25"src="http://wljy.whut.edu.cn:80/uploadfiles/word/szlj05.files/szlj055451.png">
④
<imgwidth="165"height="25"src="http://wljy.whut.edu.cn:80/uploadfiles/word/szlj05.files/szlj055476.png">