13. Select the statement that is the negation of the statement "Monica plays chess and Barbara plays tennis".  

 Solution: 
                 This problem refer the following rule:
                  ~ (p ^ q ) ≡  ~ p v ~ q                  (where: ~: Negation(NOT),    ^: AND,     v: OR)              

           if  p = "Monica plays chess"                    q ="Barbara plays tennis"           then
           ~ p = Monica does not play chess        ~ q ="Barbara does not play tennis" 

         ~ (p ^ q ) ≡  ~ p v ~ q
                         ≡ Monica does not play chess OR Barbara does not play tennis