单项选择题
A．have the lights turned on B．be rewarded with milk C．please their parents D．be praised
A．the lights were directly related to some basic "drives" B．the sight of the lights was interesting C．they need not turn back to watch the lights D．they succeeded in "switching on" the lights
A．a basic human desire to understand and control the world B．the satisfaction of certain physiological needs C．their strong desire to solve complex problems D．a fundamental human urge to display their learned skills