合同的生效
参考答案：
是指已经成立的合同开始在当事人之间发生以国家强制力保障的法律约束力，即合同发生法律效力。
A teenager is someone who can hear a song by Lady Gaga _______ three blocks away but not his mother ________ from the next door.
A．playing; calling
B．playing; called
C．played; called
D．played; calling
不准在同一垂直方向（）。
