字节对齐的问题

#include <iostream>
using namespace std;
typedef struct _A
{
char c;
int i;
} A;

typedef struct _B
{
A a;
double d;
} B;

typedef struct _C
{
A a;
char c;
} C;
int main(void)
{
cout << sizeof(A) << endl;
cout << sizeof(B) << endl;
cout << sizeof(C) << endl;
return 0;
}