Cells lining the human respiratory tract have projections that keep the passageway clean, moving debris and mucus up and out. these cellular projections are a) cilia. b) flagella. c) microtubules. d) pseudopodia.

Option A, cilia

Explanation:

Cilia are small micro-structural hairs that are attached on the bronchus of the lungs as projections. These projections have goblet cells which secrete mucus and help in trapping impurities in it thereby allowing microbes and dust free air into the lungs. It protects the lungs from all sort of invasion. Cilia's are composed of proteins and only in case of lung lining are non-moving structures.

Hence, option A is correct

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