Selling 1/3 of my holding. Does the broker sell my long term or short positions?

I've been DCA'ing over the last 1.5 years in a stock and it's gone up.

If I sell a 1/3 of that stock, does the broker sell my long term positions or short term positions or a mix? I'm tax curious. I would think they'd naturally sell the long term positions so my tax is only 15% on the gains, which is smaller than the 37% tax on short term positions.