At a Party, a Friend of Yours Keeps Popping Balloons

Multiple Choice

At a party, a friend of yours keeps popping balloons with a lighted cigarette. By the time he is about to pop the fifth balloon, you notice that your muscles tense and you squint each time the cigarette nears the balloon. Obviously,

A) the cigarette is a US.
B) the popping sound is a US.
C) squinting is a reinforcer.
D) shaping has occurred.