The volume of drugs consumed every year has exceeded that of petroleum in the world.

A．He was not concerned with light amplification.
B．It was easier to work with longer wavelengths.
C．His partner Schawlow had already begun to work on laser.
D．The laser had already been developed.

A．The researchers' notebooks were lost.
B．Several people were developing the idea at the same time.
C．No one claimed credit for the development until recently.
D．The research work is still incomplete.