This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Histo Pathology, Subject name: Pathology
Percentage of formalin used for histopathological preservative? 
Select and explain. 
A. 5%, B. 10%, C. 20%, D. 40%
B: 10%
Answer: B: 10% 
Explanation: 2Ans. (b) 10%Ref: Handbook of Histopathological and Histochemical Techniques 3rd ed. / 40-45* Formalin is generally the preferred fluid for fixation and is widely used. Formalin is often sold as 37-40% aqueous Formaldehyde.* To make a solution of 10% Formalin, nine parts of water are added to one part of 40% (aqueous) Formaldehyde.* Therefore, a 10% solution of Formalin is the equivalent of a 4% solution of Formaldehyde.* Formaldehyde is a gas produced by the oxidation of methyl alcohol, whereas 100% Formalin is a saturated solution of this gas in water.Other Histopathological Preservatives:* Glutaraldehyde: 25%-for electron microscopy (with osmium tetra-chloride)* Acetaldehyde acrolein* Mercuric chloride* Potassium dichromate