How to connect 2 QCombobox by focus?

[vinh]

I'm have 2 QCombobox, cbxA and cbxB.
I'm try
connect( ui->qlineedited, SIGNAL( returnPressed() ), ui->cbxA, SLOT( setFocus() ) ); code run OK, cbxA is focused.

connect( ui->cbxA, SIGNAL( ???() ), ui->cbxB, SLOT( setFocus() ) ); I don't know SIGNAL after press enter in QCombobox?
I'm try so much ex: ui->cbxA->lineEdit() but cbxA don't have QLineEdit, because don't need it,...
How to connect QCombobox to focus it?