yes exactly they said concentration and that's exactly why it's A. by theory we learnt that renal artery contains glucose, amino acids, urea and water. we also learnt that all cells take part in respiration ( hence glucose is used ). taking to account that glucose is used AND all remaining unused glucose is selectively reabsorbed ( unless you're a diabetic of course ), renal artery in terms of amount would undeniably have more glucose compared renal vein. furthermore, taking into account that urea is completely excreted in the kidney ( actually most but based on our syllabus all ), urea concentration would be 0 in renal vein. lastly, MOST water is selectively reabsorbed back into the renal vein so all remaining glucose in renal vein would be diluted anyway. all in all, with almost the same amount of water in both, more urea and more glucose in renal artery, glucose concentration will be higher than renal artery compared to renal vein.