This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
Which of the following layer of the tear film is the superficial layer which is in contact with air? 
Select and explain. 
A. Lipid layer, B. Aqueous layer, C. Mucinous layer, D. None of the above
A: Lipid layer
Answer: A: Lipid layer 
Explanation: Layers of the tear film: The tear film is composed of three primary layers; The superficial lipid layer is a monomolecular film at the air-tear interface derived from meibomian glands. It is thought to retard evaporation and form a wateight seal when the lids are closed. The middle aqueous layer is elaborated by the major and minor lacrimal glands and contains water-soluble substances (salts and proteins). The deep mucinous layer is composed of glycoprotein and overlies the corneal and conjunctival epithelial cells. It is elaborated by goblet cells of the conjunctiva and also by the crypts of Henle and glands of Manz. Ref: Nijm L.M., Garcia-Ferrer F.J., Schwab I.R., Augsburger J.J., Correa Z.M. (2011). Chapter 5. Conjunctiva & Tears. In P. Riordan-Eva, E.T. Cunningham, Jr. (Eds), Vaughan & Asbury's General Ophthalmology, 18e.