单项选择题

It can be inferred from the passage that the gases of the Earth's atmosphere are contained by ______.

A．a closed surface
B．the gravity of the planet
C．the field of space
D．its critical point
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单项选择题
The babies Would smile and bubble at the lights because ______.
A．they succeeded in "switching on" the light
B．the sight of the lights was interesting
C．they need not turn back to watch the lights
D．the lights were directly related to some basic "drives”
单项选择题
In Papousek's experiment babies make learned movements of the head in order to ______.
A．be praised
B．please their parents
C．be rewarded with milk
D．have the lights turned on
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