C++ Language question

I have tried to build the following code (NOT generated by app wizard) under VC++ but I encounter linker error. Would you please help me out?
 
// Here are all the code ------------------------------
 
#include <afxwin.h>
 
class box : public CWinApp
{
public:
    box(){}
    BOOL InitInstance();
};
 
BOOL box::InitInstance()
{
    AfxMessageBox("This is a message box.", MB_YESNO|MB_ICONQUESTION);
        return TRUE;
}
 
box theApp;
 
//end ---------------------------------------------
 
The linker errors are :
 
Linking...
nafxcw.lib(thrdcore.obj) : error LNK2001: unresolved external symbol __endthreadex
nafxcw.lib(thrdcore.obj) : error LNK2001: unresolved external symbol __beginthreadex
libc.lib(crt0.obj) : error LNK2001: unresolved external symbol _main
box.exe : fatal error LNK1120: 3 unresolved externals
Error executing link.exe.
box.exe - 4 error(s), 0 warning(s)
 
Thanks a lot.
 
ka ming
 
(END)
chankmAsked:
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yonatCommented:
Under the Build menu, select Settings.
In the General tab, change the combo box from "Not using MFC" to
"Using MFC as a shared DLL".
Go to the C++ tab. In the combo box, select "Code generation".
Change the combo box at the right from "Single threaded" (or
"Debug single threaded" in the Debug configuration) to "Multi-
threaded DLL" (or "Debug Multithreaded DLL" in the Debug
configuration).

Hope this helps.
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chankmAuthor Commented:
Could you explain why such simple program need a multithreaded DLL?
By the way, I think yonat's answer is quite good.
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