|  4.  | 
                        
                           Which of the following statements are true of positional parameters? 
                            Please select all the correct answers.
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                         A.  | 
                        
                           They can be checked for validity before proceeding with a script.
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                           B.
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                           They are referenced by a number preceded by a dollar sign.
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                           C.
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                           They can be used in place of environment variables.
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                           D.
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                           They can be used within any command in a shell script.
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