For gas exchange to be efficient, the respiratory membrane must be a. at least 3 micrometers thick b.0.5 to 1 micrometer thick c. between 5 and 6 micrometers thick d. the thickness of the respiratory membrane is not important in the efficiency of gas exchange.

The correct answer is b.0.5 to 1 micrometre thick.
Usually, the respiratory membrane is about 0.6 micrometres thick. This membrane consists of the alveolar epithelial cells, the capillary endothelial cell, and basement membranes which are formed by the alveolar and capillary cells.