Disabling other dropboxes on selection of value in one
I am trying to write a function that will:

 - Disable the submit button when no option in a dropbox is selected (ie empty string is   selected). Enable all dropboxes.
 - On user selection of an option in a dropbox (that is not the empty
   string), disable all other drop boxes and enable the submit button.

However, as a beginner at JavaScript I have no idea how I would begin this. How would I write such code?

So my simplified HTML would be as follows : 

    <form id="form" name="form" method="post" action="confirmation.cshtml">
       <select name="start_to_end_time" class="start_to_end_time">
          <option value="1">1</option>
          <option value="2">2</option>
       </select>
    
       <select name="start_to_end_time" class="start_to_end_time">
          <option value="3">3</option>
          <option value="4">4</option>
       </select>
       
       <button type="submit">Submit</button>

How can I write code that fulfills this function?
Disabling other dropboxes on selection of value in one
===

I am trying to write a function that will:

 - Disable the submit button when no option in a dropbox is selected (ie empty string is   selected). Enable all dropboxes.
 - On user selection of an option in a dropbox (that is not the empty
   string), disable all other drop boxes and enable the submit button.

However, as a beginner at JavaScript I have no idea how I would begin this. How would I write such code?

So my simplified HTML would be as follows : 

    <form id="form" name="form" method="post" action="confirmation.cshtml">
       <select name="start_to_end_time" class="start_to_end_time">
          <option value="1">1</option>
          <option value="2">2</option>
       </select>
    
       <select name="start_to_end_time" class="start_to_end_time">
          <option value="3">3</option>
          <option value="4">4</option>
       </select>
       
       <button type="submit">Submit</button>

How can I write code that fulfills this function?