The principal reasons for which gills were replaced in terrestrial animals are
I-air is less buoyant than water.
II-for secretion in intrapleural fluid.
III-water diffuses into air through evaporation.

Question

Quizplus · Accepted Answer

The first reason, I, is correct: air is less buoyant than water, meaning it does not provide the same support to the body as water, so gills would not be effective. 
The third reason, III, is also correct: water diffuses into air through evaporation, so the moist surfaces of gills that are necessary for gas exchange would dry out quickly in a terrestrial environment. 
The second reason, II, is not a valid reason for the evolution of lungs, as intrapleural fluid is not related to gas exchange.

The Principal Reasons for Which Gills Were Replaced in Terrestrial