Solve this error @compile Qt Project just a Demo Under Linux

it is not once that i meet this error ,what ? all the same as the last ?

../../../qtsdk-2010.05/qt/include/QtCore/qstring.h:60:22: fatal error: string: No such file or directory
compilation terminated.
make: *** [test.o] Error 1

Yes,it is a Qt project and wrong with that.
OK ?
where is wrong ?
my code:

#include<QApplication>
#include<QLabel>

int main(int argc,char * argv[]){
  QApplication app(argc,argv);
QLabel * lbl= new QLabel("Hello Qt");
lbl->show();
return app.exec();
}

No error is written  anywhere.
and ?
  env  ?

export QTDIR=/home/ubuntu/qtsdk-2010.05/qt
export PATH=$QTDIR/bin:$PATH
export MANPATH=$QTDIR/doc/man:$MANPATH
export LD_LIBRARY_PATH=$QTDIR/lib:$LD_LIBRARY_PATH
export QMAKESPEC=$QTDIR/mkspecs/linux-g++

it is located under user 's :.bashrc

OK  .no wrong .
but.................
this error is error.....
let 's see again ...what ?string.h ?that is a C++ file ?
yes! my file name is test.c and not test.cpp
that is the reason.ok let's change.