Why should there be only one variable in an experiment

Which two lines in this excerpt from arthur conan doyle's "the contest" suggest that emperor nero was ruthless? each choice is in brackets. [an hour later the shepherd was well on his way to his mountain home, and about the same time the emperor, having received the chaplet of olympia for the incomparable excellence of his performance, was making inquiries with a frowning brow as to who the insolent person might be who had dared to utter such contemptuous criticisms.] [“bring him to me here this instant,” said he, “and let marcus with his knife and branding-iron be in attendance.”] [“rumours! ” cried the angry nero. “what do you mean, arsenius? i tell you that the fellow was an ignorant upstart, with the bearing of a boor and the voice of a peacock.] [i have half a mind to burn their town about their ears so that they may remember my visit.”] [“i conquered! you are mad, arsenius. what do you mean? ”] [it is whispered that for once the great god pan has condescended to measure himself against a mortal.”]