How to set the limit of selected choices?

I use javascript to set a Multiple Answer question. I set that subjects can only select 5 choices. When they select 5 choices, the unselected choices will be disabled. When I preview the questionnaire, I find that the code works in the browser view, but it fails to work in the mobile phone view. What's wrong with it?
How can I solve this problem?
Thanks a lot.

Here is the code I used:
> this.questionclick = function(event,element){
> var selectedchoice=this.getSelectedChoices();
> if(selectedchoice.length==3){
> jQuery("#"+ this.questionId +" input:checkbox:not(:checked)" ).attr("disabled", true);
> }else{
> jQuery("#"+ this.questionId +" input:checkbox:not(:checked)" ).attr("disabled", false);
> };
> };