This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal tract, Subject name: Pharmacology
All the following are bulk-forming laxatives except 
Select and explain. 
A. Polycarbophil, B. Psyllium, C. Methylcellulose, D. Glycerin
D: Glycerin
Answer: D: Glycerin 
Explanation: Glycerin is a hyperosmotic laxative.Bulk formers: Polycarbophil, psyllium, and methylcellulose preparations. the dosage varies, consult product labeling.Hyperosmotics: Glycerin, 2-3 g suppository per rectum daily. Polyethylene glycol 3350 (powder), 17 g dissolved in 4-8 oz of beverage dailyStool softeners: Docusate sodium, 50-500 mg daily. Docusate calcium, 240 mg dailyStimulant laxatives: Bisacodyl, 5-15 mg daily. Senna, the dosage varies, consult product labelingRef: Katzung 13th ed. Pg 1090