问C语言中Linking... ex1.obj : error LNK2001: unresolved external symbol _withdrawal Debug/ex1.exe

编译器说下面我注释的那行
switch(menu())
{
case 1:open_account();break;
case 2:lancel_account();break;
case 3:withdrawal();break; /*这个函数没有定义*/
case 4:deposit();break;
case 5:query();br