下面程序的执行结果是( )。 #include＜iostream＞ using namespace std; class building { public:building() {}building(unsigned stories, float breadths, float lengths){ story = stories; breadth = breadths; length = lengths;}void getstory(void){ cout＜＜"story is:"＜＜story＜＜endl;}void getarea(void){ area = length*breadth*story;cout＜＜"area is:"＜＜area＜＜endl;} private:unsigned story;float length;float breadth;float area; }; void main(void) {building b1,b2;building b3(10u,16.6,58.8);b1.getstory();b1.getarea();b2.getstory();b2.getarea();b3.getstory();b3.getarea();}

A．story is:0area is:0story is:0area is:0story is:10area is:9760.8

B．story is:nullarea is:nullstory is:nullarea is:nullstory is:10area is:9760.8

C．前两个对象输出的结果是不定的，后一个对象的结果正确

D．前两个对象没有初值，因此程序编译时出错

电晕对定子线棒有什么危害？