The rigid tube used to suction the upper airway is known as the:

Streams are naturally occurring open channels with varying uniformity of cross section and ___________________.

Fill in the blank(s) with the appropriate word(s).

Also known as the visceral peritoneum, the ____ of the alimentary canal wall secretes serous fluid to keep other organs from sticking to the structures of the alimentary canal. 

A. mucosa B. submucosa C. muscular layer D. serosa