Disable Input with Dropdown

HTML

<div>
  <select id="val_equipfc" name="val_equipfc" required>
    <option value="A">Yes</option>
    <option value="B">No</option>
    <option value="C">Maybe</option>
  </select>
  
  <label><span style="color:red">With Equipment?</span></label>
</div>

<div>
  <input type="text" id="val_equipnofc" name="val_equipnofc">
  <label><span style="color:red">*</span>Number of Equipment/s</label>
</div>

JavaScript

document.getElementById ("val_equipfc").addEventListener ("change", 
	function(obj) {
  	var input = document.getElementById("val_equipnofc");
    input.disabled = (this.value == "B" || this.value == "C");
  }, false);