RB751G-2Hnd antenna type

Hello,
I am a little confused with what antenna type should I use, when I want to use ALL 3 internal antennas only.

The manual says :

RB 751U has 3 builtin wireless antennas
Chain0
one antenna for TX
one antenna for RX
Chain1
one antenna for TX/RX
MMCX connector for external antenna

So my question is what is the correct setup of antenna type:

Should I use
antenna a or
rx-a/tx-b or
rx-b/tx-a ?

Thank you very much for the clarification