$6.$ As in the last question, assume a zero dollar risk$-$free rate and a negative risk$-$free rate on cubits. $($You can assume these rates remain constant.$)$ You can trade American calls and puts on cubits with one year maturity and strike of $$100$ per cubit. Assuming perfect markets$($including no transaction costs associated with short selling$),$ are there any circumstances under which you would exercise the American call prior to maturity? How about the American put? You do not need to give the full arbitrage argument, but you should provide a clear brief explanation.