Three of the following contributed to the end of the pax romana (3rd century a. which one did not?
a. a 20-year plague weakened the roman army and demoralized the civilian population.
b. barbarian invasions caused widespread destruction and loss of life in several roman provinces.
c. the empire was divided into eastern and western halves by two rival emperors.
d. frequent revolts by units of the roman army led to the murder of many roman emperors.