Which period lasted longest

A．has brought about many social and economic problems
B．has brought about many unsolvable social and economic problems
C．has brought about technological problems in the economic society
D．has brought about many social and economic problems for us to solve

A．We are living in a transformed world, so it's necessary for us to raise the education standard.
B．The transformation calls for a better understanding of the happening of our time and an anticipation of major techno-logical tendency.
C．The transformation calls for enlarging colleges and universities and helping policy-makers make better policies.
D．Problems resulting from transformation are so hard to solve that we must mobilize a wide range of talent to solve them.