Which period lasted longest

A．Speak.
B．Write.
C．Explanation.
D．Talk.

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.