You are answering a USMLE-style medical question. Question: A researcher is conducting an experiment on the mouse kidney to study the relative concentrations between the tubular fluid and plasma of a number of substances along the proximal convoluted tubule. Based on the graph shown in figure A, which of the following best describes the tubular fluid-to-plasma concentration ratio of urea? Options: A) B B) C C) D D) E Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.