How to tell ethminer which GPU to use

Hi,

I have an onboard GPU and a GeForce GTX card in my system. This is how I start the miner:

ethminer.exe -G -t 1 -F "http://lcalhost:8545"
Found suitable OpenCL device [Intel(R) HD Graphics 4600] with 1494849946 bytes
f GPU memory

But no matter how I choose -t (I've tried 0, 1 , 2 and even 3),
it always chooses the Intel onbard chip as GPU. So how do I tell ethminer to use my GTX card?