This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anaesthesia
True about Boyle's apparatus – a) Continuous flow machineb) Liquid anesthetic vapours not usedc) Resistance very highd) Resistance low 
Select and explain. 
A. b, B. c, C. ad, D. ab
C: ad
Answer: C: ad 
Explanation: Boyle's machine operates on the continuous flow principle, where gas flows all the time during inspiration and expiration.
Vaporizer contain a volatile inhalation anesthetic agent in liquid form which is vaporized before being mixed with gaseous mixture.
Resistance to intermittent gas flow should be very low.