Rank the various Opioids according to their duration action from the longest to the shortest :
a. alfentanil , fentanil , remifentanil , morphine
b. morphine , alfentanil , fentanil , remifentanil
c. morphine , alfentanil , remifentanil , fentanil
d. morphine , fentanil , alfentanil , remifentanil
Select one of the following: