a. becomes equal to the external environmental air pressure by the action of the respiratory muscles
b. is less than the atmospheric pressure during inspiration
c. is less than atmospheric pressure during inhalation
d. increases when the diaphragm and external intercostal muscles contract
e. is the difference between the pressure in the pleural cavity and that within the lung alveoli.
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It seems to me that both a and d are incorrect. A-the respiratory muscles instead act to create differences between the intrathoracic and external pressures. D-When the diaphragm contracts, increasing the volume of the thorax, intrathoracic pressure actually decreases.
B and C both appear to be correct to me, as intrathoracic pressure is lower than atmospheric pressure during inspiration and inhalation. Not seeing where this question comes from, I wonder if you are able to give more than one answer, or if you are supposed to give 1 only.