单项选择题

以下程序的输出结果是( )。#include <iostream.h>void main(){ int a=0,i; for(i=1 ;i<5;i++) { switch(i) { case 0: case 3: a+ =2; case 1: case 2: a+ =3; default: a+ =5; } } cout<<a<<end1; return;}

A.31
B.13
C.10
D.20