[3.15]以下程序的运行结果是()。#includeusingnamespacestd;classA{public:voidset(inti,intj){ x=i;y=j;}intget._y(){returny;}private:intx,y;};classbox{public:voidset(intl,intw,ints,intp){length=l;width=w;label.set(s,p);}intget_area(){returnlength*width;}private:intlength,width;Alabel;};intmain(){boxb;b.set(4,6,1,20);cout

A. 24
B. 4
C. 20
D. 6