It spread through trainers and new innovations. Most popularly, the invention of the printing press.
<span>Russian physiologist Ivan Pavlov originated the study of classical conditioning while studying the salivation of dogs and finding that they would begin salivating even when they were not supposed to. Pavlov's classic experiment consisted of his paring of the ticking sound of a metronome with the presentation of food to see if the dogs would eventually salivate to just the sound of the ticking metronome.
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I believe you must drive to the right of the median. (I’m not 100% positive)