In his research at Columbia University, Charles Townes worked with all of the following EXCEPT _________.

A．A device for stimulating atoms and molecules to emit light.
B．An atom in a high-energy state.
C．A technique for destroying atoms or molecules.
D．An instrument for measuring light waves.

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