The Following Is a List of Features Associated with Respiratory

The following is a list of features associated with respiratory systems.
1) Countercurrent exchange
2 ) Cross-current exchange
3 ) Tidal ventilation
4 ) Unidirectional flow of respiratory medium across the respiratory surface
5 ) Compliant air sacs in association with a stiff multitubular gas exchange surface
6 ) Compliant lungs with many small chambers
Which collection of features would provide the most efficient gas exchange mechanism for an air breather?

A) 2, 3, 6.
B) 1, 4, 6.
C) 2, 3, 5.
D) 1, 4, 5.
E) 1, 3, 5.