From the preceding results we can say that triangular arbitrage is approximately ruled out as a source of riskless profit.$$ However, we need to add the word$$ "approximately" since

A$.$

trading hours across currency markets are not synchronized.$$

B$.$

transitory but very$$ short$-$lived profit opportunities do arise.

C$.$

collusion among traders can allow for some triangular arbitrage.

D$.$

All of the above.