In which trimester does the following occur? the respiratory system becomes mature.

Not sure, but I believe the 2nd or 3rd Trimester :)

Hope I helped!

Iwould put 'think' in the blank

Smoking damages the respiratory system by inhalation of a foreign substance, that is absorbed and residue is left behind on our lungs sponge like substance. the gas that is inhaled also increase your blood pressure and dilates your blood vessels. also the process of inhaling and exhaling the smoke it causes irritation to the throat and mouth. smoking can cause lung cancer and other respiratory and circulatory diseases.