题目

There’s talk today about how as a society we’ve become separated by colors, income, city vs suburb, red state vs blue．But we also divide ourselves with unseen dotted lines．I’m talking about the property lines that isolate us from the people we are physically closest to: our neighbors． It was a disaster on my street, in a middle-class suburb of Rochester Town, several years ago that got me thinking about this．One night, a neighbor shot and killed his wife and then himself; their two middle-school children ran screaming into the night．Though the couple had lived on our street for轻质弹簧吊着小球静止在如图所示的A位置，现用水平外力F将小球缓慢拉到B位置，此时弹簧与竖直方向的夹角为θ，在这一过程中，对于整个系统，下列说法正确的是（　　）A．系统的弹性势能不变B．系统的弹性势能增加C．系统的机械能不变D．系统的机械能增加