This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endometriosis and Infertility, Subject name: Gynaecology & Obstetrics
In semen banks, semen is preserved at low temperature using: 
Select and explain. 
A. Dry ice, B. Deep freeze, C. Liquid nitrogen, D. Liquid air
C: Liquid nitrogen
Answer: C: Liquid nitrogen 
Explanation: Ans. is c, i.e. Liquid nitrogenRef: Jeffcoate 7th/ed, p723Cryopreservation of semen:Involves cooling of embryos in the pronucleate stage or early cleavage stage to very low temperature in the presence of cryoprotectants such as:* I2 - Propanediol (Iodine)* Glycerol* Dimethyl sulphoxide (DMSO) with sucrose.* They are then stored in liquid nitrogen till required.Q* Over half the embryos survive thawing process.