Which line from Act 2 of The Monsters Are Due on Maple Street helps to distinguish this teleplay as a work of science fiction rather than a work of fantasy fiction?

"Go ahead, Steve. What kind of ‘radio set’ you workin' on? I never seen it. Neither has anyone else. Who you talk to on that radio set? And who talks to you?"
A figure has suddenly materialized in the gloom and in the silence we can hear the clickety-clack of slow, measured footsteps on concrete as the figure walks slowly toward them.
"We're all scared of the same thing. I was just tryin' to . . . tryin' to protect my home, that's all! Look, all of you, that's all I was tryin' to do."
"Their world is full of Maple Streets. And we'll go from one to the other and let them destroy themselves."