I made a short multiple choice quiz in ruby using erb in an rhtml page.

I have an array of arrays:

@allanswers = [["thailand", "jamaica", "nairobi", "indonesia"],
    ["lincoln", "kennedy", "roosevelt", "reagan"],
    ["brazil", "canada", "columbia", "peru"],
    ["nelson", "bart", "kennedy", "jack"]]

and then I want to have the multiple choice options in a random order.
Meaning that every time a person refreshes the page, the order of the
choices changes.

<% 4.times do |num|%>
<INPUT type="radio" name="answer" value="<%=first =
@allanswers[@questionNum][rand(num)]%>"> <%=first%> <BR>
<%end%>

where @allanswers is the array of arrays.
@questionNum is an iterator in the code elsewhere

The obvious problem with this method is that the rand(num) ends up
giving duplicate options. What elegant solution can you think for this
problem? Basically, I just want to get random but unique numbers.

Thank you for reading.
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