一枚硬币投掷两次,令<imgwidth="32"height="25"v:shapes="_x0000_i1040"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb06.files/image032.png">“第<imgwidth="9"height="17"v:shapes="_x0000_i1041"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb06.files/image034.png">次正面朝上”<imgwidth="55"height="21"v:shapes="_x0000_i1042"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb06.files/image036.png">,则“至多有一次正面朝上可表示为()
①
<imgwidth="80"height="26"v:shapes="_x0000_i1043"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb06.files/image038.png">
②
<imgwidth="33"height="26"v:shapes="_x0000_i1044"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb06.files/image040.png">
③
<imgwidth="35"height="27"v:shapes="_x0000_i1045"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb06.files/image042.png">
④
<imgwidth="50"height="25"v:shapes="_x0000_i1046"src="http://wljy.whut.edu.cn/uploadfiles/word/xxdsb06.files/image044.png">