题目

Carbon dioxide, which makes a ______ between us and the sun, prevents heat from getting out of the atmosphere easily, so the earth is becoming warmer.  A. difference            B. comparison           C. connection           D. barrier 答案：D收音机上调节音量的旋钮实际上是一个旋钮型变阻器如图所示，若接线片A、B已接入了电路，则顺时针转动旋钮触片时，收音机的音量将______．（选填：“不变”、“变大”或“变小”）