Bjorn's alarm clock makes a faint clicking noise a second before the alarm goes off. at first, the clicking noise doesn't wake bjorn, but the alarm does. after a couple weeks, he awakens as soon as he hears the clicking noise. the clicking noise is a(n)

unconditioned stimulus

Explanation:

Unconditioned stimulus -

It is the type of stimulus which automatically , unconditionally triggers any response , is referred to as unconditioned stimulus .  

The example of an unconditioned stimulus , is the smell of very delicious food , automatically makes a person hungry.  

Hence, from the question , as the alarm clock act as a unconditioned stimulus.  

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