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Choice B is correct. It's given that the graph shows the possible combinations of the number of pounds of tangerines, $x$, and the number of pounds of lemons, $y$, that could be purchased for $\$18$ at a certain store. If Melvin purchased lemons and $4$ pounds of tangerines for a total of $\$18$, the number of pounds of lemons he purchased is represented by the y-coordinate of the point on the graph where $x=4$. For the graph shown, when $x=4$, $y=10$. Therefore, if Melvin purchased lemons and $4$ pounds of tangerines for a total of $\$18$, then he purchased $10$ pounds of lemons.

Choice A is incorrect. This is the number of pounds of tangerines Melvin purchased if he purchased tangerines and $4$ pounds of lemons for a total of $\$18$.

Choice C is incorrect. This is the number of pounds of lemons Melvin purchased if he purchased lemons and $2$ pounds of tangerines for a total of $\$18$.

Choice D is incorrect. This is the number of pounds of lemons Melvin purchased if he purchased lemons and $1$ pound of tangerines for a total of $\$18$.