The computer that was infected from outside the network could not be any of the computers that received the virus from another computer because it is impossible to get infected twice - the rules tell us each computer received the virus only once. The rules tell us that some computer transmitted the virus to R and S, this rule eliminates R and S as the original source of the virus because some other computer on the network transmitted the virus to them. R/T transmitted the virus to Q and T/U transmitted the virus to P - these rules eliminate Q and P as well since they are both the recipients of the virus from another computer on the network. The only answer choice remaining is T that could be the original computer infected from outside the network.
