The function only makes sense for
D) All positive real numbers
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It also makes sense for x=0, which would make the domain "all non-negative real numbers." However, that is not a choice.
As shown, the function is defined for all real numbers, which includes all fractions and all integers. However, in the real world, you won't be driving the truck a negative number of miles and you won't be charged less than $15 per day. So, in the real world, the practical domain of the function is as described above.
