Doubt -- Structures

Pgm:
1. #include<stdio.h>

2. struct XXX
3. {
4. int a;
5. float b;
6. char s;
7. }structure;


8. void main()
9. {
10.
11. printf("Sizeof structure = %d\n",sizeof(structure));
12.}

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Ans1: Sizeof structure = 12 (Compiling as shown above)
Ans2: sizeof structure = 8 (when comment lineno:6)
Ans3: sizeof structure = 1 (when comment linenos:4&5)


Ans4: sizeof structure = 12 (When change lines 4 only
line 4: int a:4;)

Ans5: sizeof structure = 8 (When change lines 4 &5
line 4: int a:4;
line 5: float b:4;)

What it means (int a:4)???

COMPILER USED : Microsoft Visual C++ 6.0