Javascript 如何根据选择的选项更改消息

因此，当用户选择一个选项时，我需要更改一条消息。但我似乎无法改变信息。这是我的

HTML:

更新你的功能

function changingMessage() {
var b = a.options[a.selectedIndex].value;
if (b != c) {
            document.getElementById("message").innerHTML = "Usage has increased" + b + "year over year";
        }
        else { document.getElementById("message").innerHTML ="Usage has decreased" + c + "year over year";
        }
    }

并在选择框上添加onchange事件

<select style="border: 2px solid black;" name="comp" onchange="changingMessage()" id="comp">

主要账单：C#000000


比较条例草案：
C#123456
C#999999
C#987654

function changingMessage() {
var b = a.options[a.selectedIndex].value;
if (b != c) {
            document.getElementById("message").innerHTML = "Usage has increased" + b + "year over year";
        }
        else { document.getElementById("message").innerHTML ="Usage has decreased" + c + "year over year";
        }
    }

<select style="border: 2px solid black;" name="comp" onchange="changingMessage()" id="comp">