Sentence 1: Po's internet wasn't working. 
 Sentence 3: But his antivirus software was no help 
 Sentence 4:  His roommate accidentally turned off the antivirus software 
 Sentence 5:  And when Po used his computer next, the internet was working again 
 Option 1: He did not have any antivirus software on his computer. 
 Option 2: So he was sure his computer was infected by a virus.