$($b$)$ Now suppose that both individuals have concave utility in money, with uA$($x$)=$ ln$($xA$)$ and uB$($x$)=$ ln$($xB$),$ where ln$(.)$ is the natural logarithm function. Recall that d dx ln$($x$)=1$ x $.$ i$.$ What are all the Pareto efficient allocations in X$?$