求解答為什么結(jié)果老是顯示 NaN？

?<script?type="text/javascript">
???function?count(){
???????var?txt1?=?document.getElementById("txt1");?
???????var?a?=?parseInt(txt1.value);
???????alert(a);
????//獲取第一個輸入框的值
???		//alert(txt1.value);
????var?txt2?=?document.getElementById("txt2").value;
????var?b?=?parseInt(txt2.value);
	//獲取第二個輸入框的值
	//獲取選擇框的值
????var?select?=?document.getElementById("select").value;
????var?c=?'';
	//獲取通過下拉框來選擇的值來改變加減乘除的運算法則??
????switch(select){
????????case?"+":
???????????c=?a?+?b;
????????????alert(c);
????????????break;
????????case?"-":
????????????c?=?a?-?b;
????????????break;
????????case?"*":
???????????c?=?a?*?b;
????????????break;
????????case?"/":
???????????c?=?a?/?b;
????????????break;
????????????
????}
????//設(shè)置結(jié)果輸入框的值?
?????document.getElementById("fruit").value?=??c;
???}